<!DOCTYPE html>
<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3751">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>
    CSS.registerProperty({
        name: '--length',
        syntax: '<length>',
        initialValue: '0px',
        inherits: false
    });
</script>
<style>
    @keyframes font_size_animation {
        from {
            font-size: 10px;
            width: 10em;
            --length: 10em;
        }
        to {
            font-size: 20px;
            width: 20em;
            --length: 20em;
        }
    }
    #target1 {
        font-size: 1px;
        animation: font_size_animation 10s -5s linear paused;
    }
</style>
<div id=target1></div>
<script>
    test(function() {
        // At the time of writing, the correct (absolute) answer is not
        // yet defined. However, whatever the correct answer is, there should
        // be no difference in 'width' and a custom property registered with
        // "<length>".
        //
        // See https://github.com/w3c/csswg-drafts/issues/3751
        assert_equals(getComputedStyle(target1).getPropertyValue('width'),
                      getComputedStyle(target1).getPropertyValue('--length'));
    }, 'Animating font-size handled identically for standard and custom properties');
</script>