diff --git a/lib/hyperscript/compiler.sx b/lib/hyperscript/compiler.sx
index f2b7eca0..800da93f 100644
--- a/lib/hyperscript/compiler.sx
+++ b/lib/hyperscript/compiler.sx
@@ -158,7 +158,13 @@
             (define
               scan-on
               (fn
-                (items source filter every? catch-info finally-info)
+                (items
+                  source
+                  filter
+                  every?
+                  catch-info
+                  finally-info
+                  having-info)
                 (cond
                   ((<= (len items) 1)
                     (let
@@ -174,33 +180,28 @@
                               (let
                                 ((compiled-body (if (> (len event-refs) 0) (let ((bindings (map (fn (r) (let ((name (nth r 1))) (list (make-symbol name) (list (quote host-get) (list (quote host-get) (quote event) "detail") name)))) event-refs))) (list (quote let) bindings raw-compiled)) raw-compiled)))
                                 (let
-                                  ((wrapped-body (if catch-info (let ((var (make-symbol (nth catch-info 0))) (catch-body (hs-to-sx (nth catch-info 1)))) (if finally-info (list (quote do) (list (quote guard) (list var (list true catch-body)) compiled-body) (hs-to-sx finally-info)) (list (quote guard) (list var (list true catch-body)) compiled-body))) (if finally-info (list (quote do) compiled-body (hs-to-sx finally-info)) compiled-body)))
-                                    (handler
-                                      (let
-                                        ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false)))))
+                                  ((wrapped-body (if catch-info (let ((var (make-symbol (nth catch-info 0))) (catch-body (hs-to-sx (nth catch-info 1)))) (if finally-info (list (quote do) (list (quote guard) (list var (list true catch-body)) compiled-body) (hs-to-sx finally-info)) (list (quote guard) (list var (list true catch-body)) compiled-body))) (if finally-info (list (quote do) compiled-body (hs-to-sx finally-info)) compiled-body))))
+                                  (let
+                                    ((handler (let ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false))))) (list (quote fn) (list (quote event)) (if (uses-the-result? wrapped-body) (list (quote let) (list (list (quote the-result) nil)) wrapped-body) wrapped-body)))))
+                                    (let
+                                      ((on-call (if every? (list (quote hs-on-every) target event-name handler) (list (quote hs-on) target event-name handler))))
+                                      (if
+                                        (= event-name "intersection")
                                         (list
-                                          (quote fn)
-                                          (list (quote event))
-                                          (if
-                                            (uses-the-result? wrapped-body)
-                                            (list
-                                              (quote let)
-                                              (list
-                                                (list (quote the-result) nil))
-                                              wrapped-body)
-                                            wrapped-body)))))
-                                  (if
-                                    every?
-                                    (list
-                                      (quote hs-on-every)
-                                      target
-                                      event-name
-                                      handler)
-                                    (list
-                                      (quote hs-on)
-                                      target
-                                      event-name
-                                      handler))))))))))
+                                          (quote do)
+                                          on-call
+                                          (list
+                                            (quote hs-on-intersection-attach!)
+                                            target
+                                            (if
+                                              having-info
+                                              (get having-info "margin")
+                                              nil)
+                                            (if
+                                              having-info
+                                              (get having-info "threshold")
+                                              nil)))
+                                        on-call)))))))))))
                   ((= (first items) :from)
                     (scan-on
                       (rest (rest items))
@@ -208,7 +209,8 @@
                       filter
                       every?
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :filter)
                     (scan-on
                       (rest (rest items))
@@ -216,7 +218,8 @@
                       (nth items 1)
                       every?
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :every)
                     (scan-on
                       (rest (rest items))
@@ -224,7 +227,8 @@
                       filter
                       true
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :catch)
                     (scan-on
                       (rest (rest items))
@@ -232,7 +236,8 @@
                       filter
                       every?
                       (nth items 1)
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :finally)
                     (scan-on
                       (rest (rest items))
@@ -240,6 +245,16 @@
                       filter
                       every?
                       catch-info
+                      (nth items 1)
+                      having-info))
+                  ((= (first items) :having)
+                    (scan-on
+                      (rest (rest items))
+                      source
+                      filter
+                      every?
+                      catch-info
+                      finally-info
                       (nth items 1)))
                   (true
                     (scan-on
@@ -248,8 +263,9 @@
                       filter
                       every?
                       catch-info
-                      finally-info)))))
-            (scan-on (rest parts) nil nil false nil nil)))))
+                      finally-info
+                      having-info)))))
+            (scan-on (rest parts) nil nil false nil nil nil)))))
     (define
       emit-send
       (fn
diff --git a/lib/hyperscript/parser.sx b/lib/hyperscript/parser.sx
index ce97ef38..c009c2b1 100644
--- a/lib/hyperscript/parser.sx
+++ b/lib/hyperscript/parser.sx
@@ -2543,27 +2543,55 @@
                 (let
                   ((source (if (match-kw "from") (parse-expr) nil)))
                   (let
-                    ((body (parse-cmd-list)))
+                    ((h-margin nil) (h-threshold nil))
+                    (define
+                      consume-having!
+                      (fn
+                        ()
+                        (cond
+                          ((and (= (tp-type) "ident") (= (tp-val) "having"))
+                            (do
+                              (adv!)
+                              (cond
+                                ((and (= (tp-type) "ident") (= (tp-val) "margin"))
+                                  (do
+                                    (adv!)
+                                    (set! h-margin (parse-expr))
+                                    (consume-having!)))
+                                ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
+                                  (do
+                                    (adv!)
+                                    (set! h-threshold (parse-expr))
+                                    (consume-having!)))
+                                (true nil))))
+                          (true nil))))
+                    (consume-having!)
                     (let
-                      ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
-                        (finally-clause
-                          (if (match-kw "finally") (parse-cmd-list) nil)))
-                      (match-kw "end")
+                      ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
                       (let
-                        ((parts (list (quote on) event-name)))
+                        ((body (parse-cmd-list)))
                         (let
-                          ((parts (if every? (append parts (list :every true)) parts)))
+                          ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                            (finally-clause
+                              (if (match-kw "finally") (parse-cmd-list) nil)))
+                          (match-kw "end")
                           (let
-                            ((parts (if flt (append parts (list :filter flt)) parts)))
+                            ((parts (list (quote on) event-name)))
                             (let
-                              ((parts (if source (append parts (list :from source)) parts)))
+                              ((parts (if every? (append parts (list :every true)) parts)))
                               (let
-                                ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                ((parts (if flt (append parts (list :filter flt)) parts)))
                                 (let
-                                  ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                  ((parts (if source (append parts (list :from source)) parts)))
                                   (let
-                                    ((parts (append parts (list body))))
-                                    parts)))))))))))))))
+                                    ((parts (if having (append parts (list :having having)) parts)))
+                                    (let
+                                      ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                      (let
+                                        ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                        (let
+                                          ((parts (append parts (list body))))
+                                          parts))))))))))))))))))
       (define
         parse-init-feat
         (fn
diff --git a/lib/hyperscript/runtime.sx b/lib/hyperscript/runtime.sx
index 04889425..72ea0918 100644
--- a/lib/hyperscript/runtime.sx
+++ b/lib/hyperscript/runtime.sx
@@ -66,14 +66,30 @@
 
 ;; Wait for a DOM event on a target.
 ;; (hs-wait-for target event-name) — suspends until event fires
-(define hs-init (fn (thunk) (thunk)))
+(define
+  hs-on-intersection-attach!
+  (fn
+    (target margin threshold)
+    (let
+      ((opts (dict)))
+      (when margin (dict-set! opts "rootMargin" margin))
+      (when threshold (dict-set! opts "threshold" threshold))
+      (let
+        ((cb (fn (entries observer) (let ((entry (if (> (len entries) 0) (nth entries 0) nil))) (when entry (let ((intersecting (host-get entry "isIntersecting"))) (dom-dispatch target "intersection" (dict "intersecting" intersecting "entry" entry))))))))
+        (let
+          ((observer (host-new "IntersectionObserver" cb opts)))
+          (host-call observer "observe" target)
+          observer)))))
 
 ;; Wait for CSS transitions/animations to settle on an element.
-(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+(define hs-init (fn (thunk) (thunk)))
 
 ;; ── Class manipulation ──────────────────────────────────────────
 
 ;; Toggle a single class on an element.
+(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+
+;; Toggle between two classes — exactly one is active at a time.
 (begin
   (define
     hs-wait-for
@@ -86,19 +102,21 @@
       (target event-name timeout-ms)
       (perform (list (quote io-wait-event) target event-name timeout-ms)))))
 
-;; Toggle between two classes — exactly one is active at a time.
-(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-
 ;; Take a class from siblings — add to target, remove from others.
 ;; (hs-take! target cls) — like radio button class behavior
-(define
-  hs-toggle-class!
-  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
 
 ;; ── DOM insertion ───────────────────────────────────────────────
 
 ;; Put content at a position relative to a target.
 ;; pos: "into" | "before" | "after"
+(define
+  hs-toggle-class!
+  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+
+;; ── Navigation / traversal ──────────────────────────────────────
+
+;; Navigate to a URL.
 (define
   hs-toggle-between!
   (fn
@@ -108,9 +126,7 @@
       (do (dom-remove-class target cls1) (dom-add-class target cls2))
       (do (dom-remove-class target cls2) (dom-add-class target cls1)))))
 
-;; ── Navigation / traversal ──────────────────────────────────────
-
-;; Navigate to a URL.
+;; Find next sibling matching a selector (or any sibling).
 (define
   hs-toggle-style!
   (fn
@@ -134,7 +150,7 @@
             (dom-set-style target prop "hidden")
             (dom-set-style target prop "")))))))
 
-;; Find next sibling matching a selector (or any sibling).
+;; Find previous sibling matching a selector.
 (define
   hs-toggle-style-between!
   (fn
@@ -146,7 +162,7 @@
         (dom-set-style target prop val2)
         (dom-set-style target prop val1)))))
 
-;; Find previous sibling matching a selector.
+;; First element matching selector within a scope.
 (define
   hs-toggle-style-cycle!
   (fn
@@ -167,7 +183,7 @@
             (true (find-next (rest remaining))))))
       (dom-set-style target prop (find-next vals)))))
 
-;; First element matching selector within a scope.
+;; Last element matching selector.
 (define
   hs-take!
   (fn
@@ -207,7 +223,7 @@
               (dom-set-attr target name attr-val)
               (dom-set-attr target name ""))))))))
 
-;; Last element matching selector.
+;; First/last within a specific scope.
 (begin
   (define
     hs-element?
@@ -319,7 +335,6 @@
                 (dom-insert-adjacent-html target "beforeend" value)
                 (hs-boot-subtree! target)))))))))
 
-;; First/last within a specific scope.
 (define
   hs-add-to!
   (fn
@@ -332,6 +347,9 @@
           (append target (list value))))
       (true (do (host-call target "push" value) target)))))
 
+;; ── Iteration ───────────────────────────────────────────────────
+
+;; Repeat a thunk N times.
 (define
   hs-remove-from!
   (fn
@@ -341,9 +359,7 @@
       (filter (fn (x) (not (= x value))) target)
       (host-call target "splice" (host-call target "indexOf" value) 1))))
 
-;; ── Iteration ───────────────────────────────────────────────────
-
-;; Repeat a thunk N times.
+;; Repeat forever (until break — relies on exception/continuation).
 (define
   hs-splice-at!
   (fn
@@ -367,7 +383,10 @@
               (host-call target "splice" i 1))))
         target))))
 
-;; Repeat forever (until break — relies on exception/continuation).
+;; ── Fetch ───────────────────────────────────────────────────────
+
+;; Fetch a URL, parse response according to format.
+;; (hs-fetch url format) — format is "json" | "text" | "html"
 (define
   hs-put-at!
   (fn
@@ -386,10 +405,10 @@
             ((= pos "start") (host-call target "unshift" value)))
           target)))))
 
-;; ── Fetch ───────────────────────────────────────────────────────
+;; ── Type coercion ───────────────────────────────────────────────
 
-;; Fetch a URL, parse response according to format.
-;; (hs-fetch url format) — format is "json" | "text" | "html"
+;; Coerce a value to a type by name.
+;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
 (define
   hs-dict-without
   (fn
@@ -410,27 +429,27 @@
           (host-call (host-global "Reflect") "deleteProperty" out key)
           out)))))
 
-;; ── Type coercion ───────────────────────────────────────────────
+;; ── Object creation ─────────────────────────────────────────────
 
-;; Coerce a value to a type by name.
-;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
+;; Make a new object of a given type.
+;; (hs-make type-name) — creates empty object/collection
 (define
   hs-set-on!
   (fn
     (props target)
     (for-each (fn (k) (host-set! target k (get props k))) (keys props))))
 
-;; ── Object creation ─────────────────────────────────────────────
-
-;; Make a new object of a given type.
-;; (hs-make type-name) — creates empty object/collection
-(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
-
 ;; ── Behavior installation ───────────────────────────────────────
 
 ;; Install a behavior on an element.
 ;; A behavior is a function that takes (me ...params) and sets up features.
 ;; (hs-install behavior-fn me ...args)
+(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
+
+;; ── Measurement ─────────────────────────────────────────────────
+
+;; Measure an element's bounding rect, store as local variables.
+;; Returns a dict with x, y, width, height, top, left, right, bottom.
 (define
   hs-scroll!
   (fn
@@ -443,10 +462,10 @@
           ((= position "bottom") (dict :block "end"))
           (true (dict :block "start")))))))
 
-;; ── Measurement ─────────────────────────────────────────────────
-
-;; Measure an element's bounding rect, store as local variables.
-;; Returns a dict with x, y, width, height, top, left, right, bottom.
+;; Return the current text selection as a string.  In the browser this is
+;; `window.getSelection().toString()`.  In the mock test runner, a test
+;; setup stashes the desired selection text at `window.__test_selection`
+;; and the fallback path returns that so tests can assert on the result.
 (define
   hs-halt!
   (fn
@@ -467,17 +486,13 @@
               (host-call ev "stopPropagation")))))
       (when (not (= mode "the-event")) (raise (list "hs-return" nil))))))
 
-;; Return the current text selection as a string.  In the browser this is
-;; `window.getSelection().toString()`.  In the mock test runner, a test
-;; setup stashes the desired selection text at `window.__test_selection`
-;; and the fallback path returns that so tests can assert on the result.
-(define hs-select! (fn (target) (host-call target "select" (list))))
-
 
 ;; ── Transition ──────────────────────────────────────────────────
 
 ;; Transition a CSS property to a value, optionally with duration.
 ;; (hs-transition target prop value duration)
+(define hs-select! (fn (target) (host-call target "select" (list))))
+
 (define
   hs-get-selection
   (fn
@@ -612,12 +627,12 @@
       ((all (dom-query-all (dom-body) sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
 
+
+
+
+
 (define hs-first (fn (scope sel) (dom-query-all scope sel)))
 
-
-
-
-
 (define
   hs-last
   (fn
@@ -625,7 +640,8 @@
     (let
       ((all (dom-query-all scope sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-
+;; ── Sandbox/test runtime additions ──────────────────────────────
+;; Property access — dot notation and .length
 (define
   hs-repeat-times
   (fn
@@ -643,8 +659,7 @@
               ((= signal "hs-continue") (do-repeat (+ i 1)))
               (true (do-repeat (+ i 1))))))))
     (do-repeat 0)))
-;; ── Sandbox/test runtime additions ──────────────────────────────
-;; Property access — dot notation and .length
+;; DOM query stub — sandbox returns empty list
 (define
   hs-repeat-forever
   (fn
@@ -660,7 +675,7 @@
             ((= signal "hs-continue") (do-forever))
             (true (do-forever))))))
     (do-forever)))
-;; DOM query stub — sandbox returns empty list
+;; Method dispatch — obj.method(args)
 (define
   hs-repeat-while
   (fn
@@ -673,7 +688,9 @@
           ((= signal "hs-break") nil)
           ((= signal "hs-continue") (hs-repeat-while cond-fn thunk))
           (true (hs-repeat-while cond-fn thunk)))))))
-;; Method dispatch — obj.method(args)
+
+;; ── 0.9.90 features ─────────────────────────────────────────────
+;; beep! — debug logging, returns value unchanged
 (define
   hs-repeat-until
   (fn
@@ -685,9 +702,7 @@
         ((= signal "hs-continue")
           (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))
         (true (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))))))
-
-;; ── 0.9.90 features ─────────────────────────────────────────────
-;; beep! — debug logging, returns value unchanged
+;; Property-based is — check obj.key truthiness
 (define
   hs-for-each
   (fn
@@ -707,7 +722,7 @@
                 ((= signal "hs-continue") (do-loop (rest remaining)))
                 (true (do-loop (rest remaining))))))))
       (do-loop items))))
-;; Property-based is — check obj.key truthiness
+;; Array slicing (inclusive both ends)
 (begin
   (define
     hs-append
@@ -735,7 +750,7 @@
         ((hs-element? target)
           (dom-insert-adjacent-html target "beforeend" (str value)))
         (true nil)))))
-;; Array slicing (inclusive both ends)
+;; Collection: sorted by
 (define
   hs-sender
   (fn
@@ -743,7 +758,7 @@
     (let
       ((detail (host-get event "detail")))
       (if detail (host-get detail "sender") nil))))
-;; Collection: sorted by
+;; Collection: sorted by descending
 (define
   hs-host-to-sx
   (fn
@@ -797,7 +812,7 @@
                           (dict-set! out k (hs-host-to-sx (host-get v k))))
                         (host-call (host-global "Object") "keys" v))
                       out)))))))))))
-;; Collection: sorted by descending
+;; Collection: split by
 (define
   hs-fetch
   (fn
@@ -807,7 +822,7 @@
       (let
         ((raw (perform (list "io-fetch" url fmt))))
         (cond ((= fmt "json") (hs-host-to-sx raw)) (true raw))))))
-;; Collection: split by
+;; Collection: joined by
 (define
   hs-json-escape
   (fn
@@ -838,7 +853,7 @@
         (walk)
         out)
       "\"")))
-;; Collection: joined by
+
 (define
   hs-json-stringify
   (fn
diff --git a/shared/static/wasm/sx/hs-compiler.sx b/shared/static/wasm/sx/hs-compiler.sx
index f2b7eca0..800da93f 100644
--- a/shared/static/wasm/sx/hs-compiler.sx
+++ b/shared/static/wasm/sx/hs-compiler.sx
@@ -158,7 +158,13 @@
             (define
               scan-on
               (fn
-                (items source filter every? catch-info finally-info)
+                (items
+                  source
+                  filter
+                  every?
+                  catch-info
+                  finally-info
+                  having-info)
                 (cond
                   ((<= (len items) 1)
                     (let
@@ -174,33 +180,28 @@
                               (let
                                 ((compiled-body (if (> (len event-refs) 0) (let ((bindings (map (fn (r) (let ((name (nth r 1))) (list (make-symbol name) (list (quote host-get) (list (quote host-get) (quote event) "detail") name)))) event-refs))) (list (quote let) bindings raw-compiled)) raw-compiled)))
                                 (let
-                                  ((wrapped-body (if catch-info (let ((var (make-symbol (nth catch-info 0))) (catch-body (hs-to-sx (nth catch-info 1)))) (if finally-info (list (quote do) (list (quote guard) (list var (list true catch-body)) compiled-body) (hs-to-sx finally-info)) (list (quote guard) (list var (list true catch-body)) compiled-body))) (if finally-info (list (quote do) compiled-body (hs-to-sx finally-info)) compiled-body)))
-                                    (handler
-                                      (let
-                                        ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false)))))
+                                  ((wrapped-body (if catch-info (let ((var (make-symbol (nth catch-info 0))) (catch-body (hs-to-sx (nth catch-info 1)))) (if finally-info (list (quote do) (list (quote guard) (list var (list true catch-body)) compiled-body) (hs-to-sx finally-info)) (list (quote guard) (list var (list true catch-body)) compiled-body))) (if finally-info (list (quote do) compiled-body (hs-to-sx finally-info)) compiled-body))))
+                                  (let
+                                    ((handler (let ((uses-the-result? (fn (expr) (cond ((= expr (quote the-result)) true) ((list? expr) (some (fn (x) (uses-the-result? x)) expr)) (true false))))) (list (quote fn) (list (quote event)) (if (uses-the-result? wrapped-body) (list (quote let) (list (list (quote the-result) nil)) wrapped-body) wrapped-body)))))
+                                    (let
+                                      ((on-call (if every? (list (quote hs-on-every) target event-name handler) (list (quote hs-on) target event-name handler))))
+                                      (if
+                                        (= event-name "intersection")
                                         (list
-                                          (quote fn)
-                                          (list (quote event))
-                                          (if
-                                            (uses-the-result? wrapped-body)
-                                            (list
-                                              (quote let)
-                                              (list
-                                                (list (quote the-result) nil))
-                                              wrapped-body)
-                                            wrapped-body)))))
-                                  (if
-                                    every?
-                                    (list
-                                      (quote hs-on-every)
-                                      target
-                                      event-name
-                                      handler)
-                                    (list
-                                      (quote hs-on)
-                                      target
-                                      event-name
-                                      handler))))))))))
+                                          (quote do)
+                                          on-call
+                                          (list
+                                            (quote hs-on-intersection-attach!)
+                                            target
+                                            (if
+                                              having-info
+                                              (get having-info "margin")
+                                              nil)
+                                            (if
+                                              having-info
+                                              (get having-info "threshold")
+                                              nil)))
+                                        on-call)))))))))))
                   ((= (first items) :from)
                     (scan-on
                       (rest (rest items))
@@ -208,7 +209,8 @@
                       filter
                       every?
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :filter)
                     (scan-on
                       (rest (rest items))
@@ -216,7 +218,8 @@
                       (nth items 1)
                       every?
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :every)
                     (scan-on
                       (rest (rest items))
@@ -224,7 +227,8 @@
                       filter
                       true
                       catch-info
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :catch)
                     (scan-on
                       (rest (rest items))
@@ -232,7 +236,8 @@
                       filter
                       every?
                       (nth items 1)
-                      finally-info))
+                      finally-info
+                      having-info))
                   ((= (first items) :finally)
                     (scan-on
                       (rest (rest items))
@@ -240,6 +245,16 @@
                       filter
                       every?
                       catch-info
+                      (nth items 1)
+                      having-info))
+                  ((= (first items) :having)
+                    (scan-on
+                      (rest (rest items))
+                      source
+                      filter
+                      every?
+                      catch-info
+                      finally-info
                       (nth items 1)))
                   (true
                     (scan-on
@@ -248,8 +263,9 @@
                       filter
                       every?
                       catch-info
-                      finally-info)))))
-            (scan-on (rest parts) nil nil false nil nil)))))
+                      finally-info
+                      having-info)))))
+            (scan-on (rest parts) nil nil false nil nil nil)))))
     (define
       emit-send
       (fn
diff --git a/shared/static/wasm/sx/hs-parser.sx b/shared/static/wasm/sx/hs-parser.sx
index ce97ef38..c009c2b1 100644
--- a/shared/static/wasm/sx/hs-parser.sx
+++ b/shared/static/wasm/sx/hs-parser.sx
@@ -2543,27 +2543,55 @@
                 (let
                   ((source (if (match-kw "from") (parse-expr) nil)))
                   (let
-                    ((body (parse-cmd-list)))
+                    ((h-margin nil) (h-threshold nil))
+                    (define
+                      consume-having!
+                      (fn
+                        ()
+                        (cond
+                          ((and (= (tp-type) "ident") (= (tp-val) "having"))
+                            (do
+                              (adv!)
+                              (cond
+                                ((and (= (tp-type) "ident") (= (tp-val) "margin"))
+                                  (do
+                                    (adv!)
+                                    (set! h-margin (parse-expr))
+                                    (consume-having!)))
+                                ((and (= (tp-type) "ident") (= (tp-val) "threshold"))
+                                  (do
+                                    (adv!)
+                                    (set! h-threshold (parse-expr))
+                                    (consume-having!)))
+                                (true nil))))
+                          (true nil))))
+                    (consume-having!)
                     (let
-                      ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
-                        (finally-clause
-                          (if (match-kw "finally") (parse-cmd-list) nil)))
-                      (match-kw "end")
+                      ((having (if (or h-margin h-threshold) (dict "margin" h-margin "threshold" h-threshold) nil)))
                       (let
-                        ((parts (list (quote on) event-name)))
+                        ((body (parse-cmd-list)))
                         (let
-                          ((parts (if every? (append parts (list :every true)) parts)))
+                          ((catch-clause (if (match-kw "catch") (let ((var (let ((v (tp-val))) (adv!) v)) (handler (parse-cmd-list))) (list var handler)) nil))
+                            (finally-clause
+                              (if (match-kw "finally") (parse-cmd-list) nil)))
+                          (match-kw "end")
                           (let
-                            ((parts (if flt (append parts (list :filter flt)) parts)))
+                            ((parts (list (quote on) event-name)))
                             (let
-                              ((parts (if source (append parts (list :from source)) parts)))
+                              ((parts (if every? (append parts (list :every true)) parts)))
                               (let
-                                ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                ((parts (if flt (append parts (list :filter flt)) parts)))
                                 (let
-                                  ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                  ((parts (if source (append parts (list :from source)) parts)))
                                   (let
-                                    ((parts (append parts (list body))))
-                                    parts)))))))))))))))
+                                    ((parts (if having (append parts (list :having having)) parts)))
+                                    (let
+                                      ((parts (if catch-clause (append parts (list :catch catch-clause)) parts)))
+                                      (let
+                                        ((parts (if finally-clause (append parts (list :finally finally-clause)) parts)))
+                                        (let
+                                          ((parts (append parts (list body))))
+                                          parts))))))))))))))))))
       (define
         parse-init-feat
         (fn
diff --git a/shared/static/wasm/sx/hs-runtime.sx b/shared/static/wasm/sx/hs-runtime.sx
index 04889425..72ea0918 100644
--- a/shared/static/wasm/sx/hs-runtime.sx
+++ b/shared/static/wasm/sx/hs-runtime.sx
@@ -66,14 +66,30 @@
 
 ;; Wait for a DOM event on a target.
 ;; (hs-wait-for target event-name) — suspends until event fires
-(define hs-init (fn (thunk) (thunk)))
+(define
+  hs-on-intersection-attach!
+  (fn
+    (target margin threshold)
+    (let
+      ((opts (dict)))
+      (when margin (dict-set! opts "rootMargin" margin))
+      (when threshold (dict-set! opts "threshold" threshold))
+      (let
+        ((cb (fn (entries observer) (let ((entry (if (> (len entries) 0) (nth entries 0) nil))) (when entry (let ((intersecting (host-get entry "isIntersecting"))) (dom-dispatch target "intersection" (dict "intersecting" intersecting "entry" entry))))))))
+        (let
+          ((observer (host-new "IntersectionObserver" cb opts)))
+          (host-call observer "observe" target)
+          observer)))))
 
 ;; Wait for CSS transitions/animations to settle on an element.
-(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+(define hs-init (fn (thunk) (thunk)))
 
 ;; ── Class manipulation ──────────────────────────────────────────
 
 ;; Toggle a single class on an element.
+(define hs-wait (fn (ms) (perform (list (quote io-sleep) ms))))
+
+;; Toggle between two classes — exactly one is active at a time.
 (begin
   (define
     hs-wait-for
@@ -86,19 +102,21 @@
       (target event-name timeout-ms)
       (perform (list (quote io-wait-event) target event-name timeout-ms)))))
 
-;; Toggle between two classes — exactly one is active at a time.
-(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
-
 ;; Take a class from siblings — add to target, remove from others.
 ;; (hs-take! target cls) — like radio button class behavior
-(define
-  hs-toggle-class!
-  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+(define hs-settle (fn (target) (perform (list (quote io-settle) target))))
 
 ;; ── DOM insertion ───────────────────────────────────────────────
 
 ;; Put content at a position relative to a target.
 ;; pos: "into" | "before" | "after"
+(define
+  hs-toggle-class!
+  (fn (target cls) (host-call (host-get target "classList") "toggle" cls)))
+
+;; ── Navigation / traversal ──────────────────────────────────────
+
+;; Navigate to a URL.
 (define
   hs-toggle-between!
   (fn
@@ -108,9 +126,7 @@
       (do (dom-remove-class target cls1) (dom-add-class target cls2))
       (do (dom-remove-class target cls2) (dom-add-class target cls1)))))
 
-;; ── Navigation / traversal ──────────────────────────────────────
-
-;; Navigate to a URL.
+;; Find next sibling matching a selector (or any sibling).
 (define
   hs-toggle-style!
   (fn
@@ -134,7 +150,7 @@
             (dom-set-style target prop "hidden")
             (dom-set-style target prop "")))))))
 
-;; Find next sibling matching a selector (or any sibling).
+;; Find previous sibling matching a selector.
 (define
   hs-toggle-style-between!
   (fn
@@ -146,7 +162,7 @@
         (dom-set-style target prop val2)
         (dom-set-style target prop val1)))))
 
-;; Find previous sibling matching a selector.
+;; First element matching selector within a scope.
 (define
   hs-toggle-style-cycle!
   (fn
@@ -167,7 +183,7 @@
             (true (find-next (rest remaining))))))
       (dom-set-style target prop (find-next vals)))))
 
-;; First element matching selector within a scope.
+;; Last element matching selector.
 (define
   hs-take!
   (fn
@@ -207,7 +223,7 @@
               (dom-set-attr target name attr-val)
               (dom-set-attr target name ""))))))))
 
-;; Last element matching selector.
+;; First/last within a specific scope.
 (begin
   (define
     hs-element?
@@ -319,7 +335,6 @@
                 (dom-insert-adjacent-html target "beforeend" value)
                 (hs-boot-subtree! target)))))))))
 
-;; First/last within a specific scope.
 (define
   hs-add-to!
   (fn
@@ -332,6 +347,9 @@
           (append target (list value))))
       (true (do (host-call target "push" value) target)))))
 
+;; ── Iteration ───────────────────────────────────────────────────
+
+;; Repeat a thunk N times.
 (define
   hs-remove-from!
   (fn
@@ -341,9 +359,7 @@
       (filter (fn (x) (not (= x value))) target)
       (host-call target "splice" (host-call target "indexOf" value) 1))))
 
-;; ── Iteration ───────────────────────────────────────────────────
-
-;; Repeat a thunk N times.
+;; Repeat forever (until break — relies on exception/continuation).
 (define
   hs-splice-at!
   (fn
@@ -367,7 +383,10 @@
               (host-call target "splice" i 1))))
         target))))
 
-;; Repeat forever (until break — relies on exception/continuation).
+;; ── Fetch ───────────────────────────────────────────────────────
+
+;; Fetch a URL, parse response according to format.
+;; (hs-fetch url format) — format is "json" | "text" | "html"
 (define
   hs-put-at!
   (fn
@@ -386,10 +405,10 @@
             ((= pos "start") (host-call target "unshift" value)))
           target)))))
 
-;; ── Fetch ───────────────────────────────────────────────────────
+;; ── Type coercion ───────────────────────────────────────────────
 
-;; Fetch a URL, parse response according to format.
-;; (hs-fetch url format) — format is "json" | "text" | "html"
+;; Coerce a value to a type by name.
+;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
 (define
   hs-dict-without
   (fn
@@ -410,27 +429,27 @@
           (host-call (host-global "Reflect") "deleteProperty" out key)
           out)))))
 
-;; ── Type coercion ───────────────────────────────────────────────
+;; ── Object creation ─────────────────────────────────────────────
 
-;; Coerce a value to a type by name.
-;; (hs-coerce value type-name) — type-name is "Int", "Float", "String", etc.
+;; Make a new object of a given type.
+;; (hs-make type-name) — creates empty object/collection
 (define
   hs-set-on!
   (fn
     (props target)
     (for-each (fn (k) (host-set! target k (get props k))) (keys props))))
 
-;; ── Object creation ─────────────────────────────────────────────
-
-;; Make a new object of a given type.
-;; (hs-make type-name) — creates empty object/collection
-(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
-
 ;; ── Behavior installation ───────────────────────────────────────
 
 ;; Install a behavior on an element.
 ;; A behavior is a function that takes (me ...params) and sets up features.
 ;; (hs-install behavior-fn me ...args)
+(define hs-navigate! (fn (url) (perform (list (quote io-navigate) url))))
+
+;; ── Measurement ─────────────────────────────────────────────────
+
+;; Measure an element's bounding rect, store as local variables.
+;; Returns a dict with x, y, width, height, top, left, right, bottom.
 (define
   hs-scroll!
   (fn
@@ -443,10 +462,10 @@
           ((= position "bottom") (dict :block "end"))
           (true (dict :block "start")))))))
 
-;; ── Measurement ─────────────────────────────────────────────────
-
-;; Measure an element's bounding rect, store as local variables.
-;; Returns a dict with x, y, width, height, top, left, right, bottom.
+;; Return the current text selection as a string.  In the browser this is
+;; `window.getSelection().toString()`.  In the mock test runner, a test
+;; setup stashes the desired selection text at `window.__test_selection`
+;; and the fallback path returns that so tests can assert on the result.
 (define
   hs-halt!
   (fn
@@ -467,17 +486,13 @@
               (host-call ev "stopPropagation")))))
       (when (not (= mode "the-event")) (raise (list "hs-return" nil))))))
 
-;; Return the current text selection as a string.  In the browser this is
-;; `window.getSelection().toString()`.  In the mock test runner, a test
-;; setup stashes the desired selection text at `window.__test_selection`
-;; and the fallback path returns that so tests can assert on the result.
-(define hs-select! (fn (target) (host-call target "select" (list))))
-
 
 ;; ── Transition ──────────────────────────────────────────────────
 
 ;; Transition a CSS property to a value, optionally with duration.
 ;; (hs-transition target prop value duration)
+(define hs-select! (fn (target) (host-call target "select" (list))))
+
 (define
   hs-get-selection
   (fn
@@ -612,12 +627,12 @@
       ((all (dom-query-all (dom-body) sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
 
+
+
+
+
 (define hs-first (fn (scope sel) (dom-query-all scope sel)))
 
-
-
-
-
 (define
   hs-last
   (fn
@@ -625,7 +640,8 @@
     (let
       ((all (dom-query-all scope sel)))
       (if (> (len all) 0) (nth all (- (len all) 1)) nil))))
-
+;; ── Sandbox/test runtime additions ──────────────────────────────
+;; Property access — dot notation and .length
 (define
   hs-repeat-times
   (fn
@@ -643,8 +659,7 @@
               ((= signal "hs-continue") (do-repeat (+ i 1)))
               (true (do-repeat (+ i 1))))))))
     (do-repeat 0)))
-;; ── Sandbox/test runtime additions ──────────────────────────────
-;; Property access — dot notation and .length
+;; DOM query stub — sandbox returns empty list
 (define
   hs-repeat-forever
   (fn
@@ -660,7 +675,7 @@
             ((= signal "hs-continue") (do-forever))
             (true (do-forever))))))
     (do-forever)))
-;; DOM query stub — sandbox returns empty list
+;; Method dispatch — obj.method(args)
 (define
   hs-repeat-while
   (fn
@@ -673,7 +688,9 @@
           ((= signal "hs-break") nil)
           ((= signal "hs-continue") (hs-repeat-while cond-fn thunk))
           (true (hs-repeat-while cond-fn thunk)))))))
-;; Method dispatch — obj.method(args)
+
+;; ── 0.9.90 features ─────────────────────────────────────────────
+;; beep! — debug logging, returns value unchanged
 (define
   hs-repeat-until
   (fn
@@ -685,9 +702,7 @@
         ((= signal "hs-continue")
           (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))
         (true (if (cond-fn) nil (hs-repeat-until cond-fn thunk)))))))
-
-;; ── 0.9.90 features ─────────────────────────────────────────────
-;; beep! — debug logging, returns value unchanged
+;; Property-based is — check obj.key truthiness
 (define
   hs-for-each
   (fn
@@ -707,7 +722,7 @@
                 ((= signal "hs-continue") (do-loop (rest remaining)))
                 (true (do-loop (rest remaining))))))))
       (do-loop items))))
-;; Property-based is — check obj.key truthiness
+;; Array slicing (inclusive both ends)
 (begin
   (define
     hs-append
@@ -735,7 +750,7 @@
         ((hs-element? target)
           (dom-insert-adjacent-html target "beforeend" (str value)))
         (true nil)))))
-;; Array slicing (inclusive both ends)
+;; Collection: sorted by
 (define
   hs-sender
   (fn
@@ -743,7 +758,7 @@
     (let
       ((detail (host-get event "detail")))
       (if detail (host-get detail "sender") nil))))
-;; Collection: sorted by
+;; Collection: sorted by descending
 (define
   hs-host-to-sx
   (fn
@@ -797,7 +812,7 @@
                           (dict-set! out k (hs-host-to-sx (host-get v k))))
                         (host-call (host-global "Object") "keys" v))
                       out)))))))))))
-;; Collection: sorted by descending
+;; Collection: split by
 (define
   hs-fetch
   (fn
@@ -807,7 +822,7 @@
       (let
         ((raw (perform (list "io-fetch" url fmt))))
         (cond ((= fmt "json") (hs-host-to-sx raw)) (true raw))))))
-;; Collection: split by
+;; Collection: joined by
 (define
   hs-json-escape
   (fn
@@ -838,7 +853,7 @@
         (walk)
         out)
       "\"")))
-;; Collection: joined by
+
 (define
   hs-json-stringify
   (fn
diff --git a/tests/hs-run-filtered.js b/tests/hs-run-filtered.js
index 049501f6..de8bff65 100755
--- a/tests/hs-run-filtered.js
+++ b/tests/hs-run-filtered.js
@@ -354,7 +354,26 @@ class HsResizeObserver {
   }
 }
 globalThis.ResizeObserver=HsResizeObserver; globalThis.ResizeObserverEntry=class{};
-globalThis.IntersectionObserver=class{observe(){}disconnect(){}};
+// HsIntersectionObserver — cluster-27 intersection mock. Fires the callback
+// synchronously on observe() with isIntersecting=true so `on intersection`
+// handlers run during activation. `margin`/`threshold` options are parsed
+// but ignored (tests only assert the handler fires).
+class HsIntersectionObserver {
+  constructor(cb, opts) { this._cb = cb; this._opts = opts || {}; this._els = new Set(); }
+  observe(el) {
+    if (!el) return;
+    this._els.add(el);
+    const entry = { target: el, isIntersecting: true, intersectionRatio: 1,
+                    boundingClientRect: (el.getBoundingClientRect && el.getBoundingClientRect()) || {},
+                    intersectionRect: {}, rootBounds: null, time: 0 };
+    try { this._cb([entry], this); } catch (e) {}
+  }
+  unobserve(el) { if (el) this._els.delete(el); }
+  disconnect() { this._els.clear(); }
+  takeRecords() { return []; }
+}
+globalThis.IntersectionObserver = HsIntersectionObserver;
+globalThis.IntersectionObserverEntry = class {};
 globalThis.navigator={userAgent:'node'}; globalThis.location={href:'http://localhost/',pathname:'/',search:'',hash:''};
 globalThis.history={pushState(){},replaceState(){},back(){},forward(){}};
 globalThis.getSelection=()=>({toString:()=>(globalThis.__test_selection||'')});