Make sure the start value of a transition doesn't depend on interleaving fallback styles. (#56868)

The difference between Firefox and other browsers is that other browsers
start the transition in the 50px value because of interleaving.

That's not the point of the test tho, so let's tweak it to make sure
that the expected transition runs.

Differential Revision: https://phabricator.services.mozilla.com/D277032

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2006371
gecko-commit: 4a637a959dac6bbb8b055489b3090d63351bd0c2
gecko-reviewers: layout-anchor-positioning-reviewers, dshin

Co-authored-by: Emilio Cobos Álvarez <[email protected]>

Author: moz-wptsync-bot

css/css-anchor-position/position-try-cascade.html

@@ -87,9 +87,14 @@
 
 <!-- Transitions -->
 <style>
-  #abs_transition.move {
-    top: 10px !important;
-    transition: top 1000s steps(2, start);
+  #abs_transition {
+    top: 50px;
+
+    &.move {
+      top: 10px !important;
+      transition: top 1000s steps(2, start);
+    }
+  }
 </style>
 <div class=cb>
   <div id=abs_transition class=abs></div>