diff --git a/compatibility.bs b/compatibility.bs
index 3bda92d..fba69d6 100644
--- a/compatibility.bs
+++ b/compatibility.bs
@@ -42,6 +42,21 @@ urlPrefix: https://drafts.fxtf.org/css-masking-1/
 urlPrefix: https://drafts.csswg.org/css-values-3/
   type: dfn
     text: dppx; url: #dppx
+urlPrefix: https://html.spec.whatwg.org/multipage/
+  urlPrefix: document-sequences.html
+    type: dfn
+      for: browsing context
+        text: is popup
+        text: opener browsing context
+        text: opener origin at creation
+urlPrefix: https://privacycg.github.io/nav-tracking-mitigations/
+  type: dfn
+    for: top-level traversable
+      text: bounce tracking record; url: top-level-traversable-bounce-tracking-record
+    for: bounce tracking record
+      text: initial host; url: #bounce-tracking-record-initial-host
+      text: bounce set; url: #bounce-tracking-record-bounce-set
+      text: user activation set; url: #bounce-tracking-record-user-activation-set
 </pre>
 
 <pre class="biblio">
@@ -67,9 +82,11 @@ spec:css-display-3; type:value; for:display; text:flex
 spec:css-flexbox-1; type:value; text:inline-flex
 spec:css-syntax-3; type:dfn; text:invalid
 spec:filter-effects-1; type:property; text:filter
+spec:infra; type:dfn; text:list
 spec:infra; type:dfn; text:user agent
 spec:svg2; type:dfn; text:fill
 spec:svg2; type:dfn; text:stroke
+spec:url; type:dfn; for:url; text:host
 </pre>
 
 <!-- Commented out until we know what the heck to put here:
@@ -836,6 +853,133 @@ supported on all {{Window}} objects and <code><{body}></code> elements as attrib
   </tbody>
 </table>
 
+<h2 id="sa-heuristics-section">Cookie Access Heuristics</h2>
+
+User agents are blocking third-party cookies by default while avoiding user-facing
+breakage by removing support for common web patterns, particularly login flows via
+third-party identity providers. Browsers should follow the established patterns in
+this specification in order to provide short-term cookie access to unbreak these flows.
+
+<h3 id="sa-heuristics-global-data">Global Data</h3>
+
+A <dfn>site pair</dfn> is a [=tuple=] consisting of a [=site=] <dfn for=site pair>third party site</dfn>
+and a [=site=] <dfn for=site pair>first party site</dfn>
+
+The user agent holds a <dfn>heuristic grants map</dfn> which is a [=map=] of [=site pairs=] to
+[=moments=]. The [=moment=] represents the expiration timestamp of a cookie access grant
+for [=third party site=] on [=first party site=].
+
+<h3 id="sa-heuristics-constants">Constants</h3>
+
+The <dfn>popup heuristic grant duration</dfn> is an [=implementation-defined=]
+[=duration=] that represents the length of time after the popup heuristic is
+triggered that a user agent will store the corresponding popup heuristic grant
+in its [=heuristic grants map=].
+
+Note: Most implementations use 30 days as the [=popup heuristic grant duration=].
+
+The <dfn>redirect heuristic grant duration</dfn> is an [=implementation-defined=]
+[=duration=] that represents the length of time after the redirect heuristic is
+triggered that a user agent will store the corresponding redirect heuristic grant
+in its [=heuristic grants map=].
+
+Note: Most implementations use 15 minutes as the [=redirect heuristic grant duration=].
+
+<h3 id="sa-heuristics-access-grant">Cookie Access Grant</h3>
+
+<div algorithm>
+
+To <dfn>grant access for heuristics</dfn> given a [=site=] |site|, [=site=]
+|firstPartySite|, [=moment=] |currentWallTime|, and [=duration=] |duration|,
+perform the following steps:
+
+1. Let |key| be a [=site pair=] with [=third party site=] set to |site| and
+    [=first party site=] set to |firstPartySite|.
+1. Let |expirationTime| be |duration| after |currentWallTime|.
+1. Set [=heuristic grants map=][|key|] to |expirationTime|.
+
+</div>
+
+<h3 id="sa-heuristics-access">Cookie Access Monkey Patch</h3>
+
+Issue: TODO: add monkey patch to network fetch cookie access that reads from [=heuristic grants map=].
+
+<h3 id="sa-heuristics-popup">Popup Heuristic</h3>
+
+Append the following steps to the [[!HTML]]'s <a spec="html">activation notification</a> algorithm:
+
+<div algorithm="user-activation-patch">
+
+1. Run [=detect a popup heuristic=] given <var ignore>document</var>.
+
+</div>
+
+<div algorithm>
+
+To <dfn>detect a popup heuristic</dfn> given a [=Document=] |document|,
+perform the following steps:
+
+1. Let |browsingContext| be |document|'s [=Document/browsing context=].
+1. If |browsingContext|'s [=browsing context/is popup=] is false, then return.
+1. If |browsingContext|'s [=browsing context/opener browsing context=] is null, then return.
+1. If |browsingContext|'s [=browsing context/opener origin at creation=] is null, then return.
+1. Let |firstPartySite| be the result of running [=obtain a site=] given the |browsingContext|'s
+    [=browsing context/opener origin at creation=].
+1. Let |navigable| be |document|'s [=node navigable=].
+1. If |navigable| is null, then return.
+1. Let |topDocument| be |navigable|'s [=navigable/top-level traversable=]'s
+    [=navigable/active document=].
+1. Let |origin| be |topDocument|'s [=Document/origin=].
+1. If |origin| is an [=opaque origin=] then return.
+1. Let |site| be the result of running [=obtain a site=] given |origin|.
+1. Let |currentWallTime| be |topDocument|'s [=relevant settings object=]'s
+    [=environment settings object/current wall time=].
+1. [=Grant access for heuristics=] given |site|, |firstPartySite|, |currentWallTime|, and
+    [=popup heuristic grant duration=].
+
+Issue(amaliev/3pcd-exemption-heuristics#3): TODO: Consider whether to check for third-party iframe initiators.
+
+</div>
+
+<h3 id="sa-heuristics-redirect">Redirect Heuristic</h3>
+
+Insert the following steps at the start of [[!HTML]]'s <a spec="html">load a document</a> algorithm.
+
+<div algorithm="loading-a-document-patch">
+
+1. Run [=detect a redirect heuristic=] given <var ignore>navigationParams</var>'s <var ignore>navigable</var>.
+
+</div>
+
+<div algorithm>
+
+To <dfn>detect a redirect heuristic</dfn> given a [=/navigable=] |navigable|, perform the following
+steps:
+
+1. Let |topDocument| be |navigable|'s [=navigable/top-level traversable=]'s
+    [=navigable/active document=].
+1. Let |firstPartyOrigin| be |topDocument|'s [=Document/origin=].
+1. If |firstPartyOrigin| is an [=opaque origin=] then return.
+1. Let |firstPartySite| be the result of running [=obtain a site=] given |firstPartyOrigin|.
+1. Let |bounceTrackingRecord| be |navigable|'s [=navigable/top-level traversable=]'s
+    [=top-level traversable/bounce tracking record=].
+1. Let |currentWallTime| be |topDocument|'s [=relevant settings object=]'s
+    [=environment settings object/current wall time=].
+1. Let |navigatedUrlsSet| be |bounceTrackingRecord|'s [=bounce tracking record/bounce set=].
+1. Append |bounceTrackingRecord|'s [=bounce tracking record/initial host=] to |navigatedUrlsSet|.
+1. [=list/For each=] |bounceUrl| in |navigatedUrlsSet|:
+    1. Let |site| be the result of running [=obtain a site=] given |bounceUrl|.
+    1. If |site| is [=site/same site=] to |firstPartySite|, [=iteration/continue=].
+    1. If |bounceTrackingRecord|'s [=bounce tracking record/user activation set=] does not contain |site|,
+        [=iteration/continue=].
+    <!-- TODO: Check that the user visited |firstPartySite| before |site|, to verify the
+        [A-B-A flow](https://github.com/amaliev/3pcd-exemption-heuristics/blob/main/explainer.md#scenario-c2).
+        This will require traversing the navigable's history. -->
+    1. [=Grant access for heuristics=] given |site|, |firstPartySite|, |currentWallTime|, and
+        [=redirect heuristic grant duration=].
+
+</div>
+
 <h2 id="ua-string-section">The User-Agent String</h2>
 
 The <code>User-Agent</code> header field syntax is formally defined by [[!HTTP-SEMANTICS]] and