<!DOCTYPE html>
<title>CSS Transitions Test: @starting-style inside size @container</title>
<link rel="help" href="https://drafts.csswg.org/css-transitions-2/#defining-before-change-style-the-starting-style-rule">
<link rel="help" href="https://drafts.csswg.org/css-contain-3/#animated-containers">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/css/css-transitions/support/helper.js"></script>
<div id="container" style="width: 200px">
  <div id="target" style="display: none"></div>
</div>
<style>
  #container {
    container-type: inline-size;
  }
  #target {
    transition-property: background-color;
    transition-duration: 100s;
    transition-timing-function: steps(2, start);
    background-color: lime;
  }
  @container (width > 300px) {
    @starting-style {
      #target { background-color: white; }
    }
  }
  @container (width < 300px) {
    @starting-style {
      #target { background-color: red; }
    }
  }
</style>
<script>
  promise_test(async t => {
    await waitForAnimationFrames(2);
    assert_equals(getComputedStyle(target).backgroundColor, "rgb(0, 255, 0)",
                  "No transition while display:none");
    container.style.width = "400px";
    target.style.display = "block";
    await waitForAnimationFrames(2);
    assert_equals(getComputedStyle(target).backgroundColor, "rgb(128, 255, 128)",
                  "@starting-style based on the size query evaluation from the same frame");
  }, "Triggered transition from first style update based on up-to-date container query");
</script>
