Add WPT testcase to verify that tab-size is based on the block container's <space> width, and not that of the inline element, as per spec.
Differential Revision: https://phabricator.services.mozilla.com/D125813
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1731045
gecko-commit: 51326f646b454e18ea3f4219d90b8fb3b46c7245
gecko-reviewers: emilio
diff --git a/css/css-text/tab-size/tab-size-block-ancestor-ref.html b/css/css-text/tab-size/tab-size-block-ancestor-ref.html
new file mode 100644
index 0000000..bb1cdc9
--- /dev/null
+++ b/css/css-text/tab-size/tab-size-block-ancestor-ref.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Text reference</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<style>
+div {
+ font: 16px monospace;
+ white-space: pre;
+}
+b {
+ font: bold 16px monospace;
+}
+</style>
+
+<p>Test passes if the five “A” letters below are vertically aligned with each other.
+
+<div>
+ <b>A</b>
+</div>
+<div>
+ <b>A</b>
+</div>
+<div>
+ <b>A</b>
+</div>
+<div>
+ <b>A</b>
+</div>
+<div>
+ <b>A</b>
+</div>
diff --git a/css/css-text/tab-size/tab-size-block-ancestor.html b/css/css-text/tab-size/tab-size-block-ancestor.html
new file mode 100644
index 0000000..b295382
--- /dev/null
+++ b/css/css-text/tab-size/tab-size-block-ancestor.html
@@ -0,0 +1,48 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>CSS Text level 3 Test: tab-size based on block ancestor's space</title>
+<link rel="author" title="Jonathan Kew" href="mailto:jkew@mozilla.com">
+<link rel="help" href="https://drafts.csswg.org/css-text-3/#tab-size-property">
+<link rel="match" href="tab-size-block-ancestor-ref.html">
+<meta name="assert" content="tab width is based on space character in the block ancestor">
+<style>
+div {
+ font: 16px monospace;
+ white-space: pre;
+ tab-size: 10;
+}
+.test1 span {
+ font-size: 1px;
+}
+.test2 span {
+ font-size: 10px;
+}
+.test3 span {
+ tab-size: 5;
+}
+.test4 span {
+ font-size: 10px;
+ tab-size: 2.5;
+}
+b {
+ font: bold 16px monospace;
+}
+</style>
+
+<p>Test passes if the five “A” letters below are vertically aligned with each other.
+
+<div>
+ <b>A</b><!-- indented by 10 spaces -->
+</div>
+<div class=test1>
+<span>	<b>A</b></span>
+</div>
+<div class=test2>
+<span>	<b>A</b></span>
+</div>
+<div class=test3>
+<span>		<b>A</b></span>
+</div>
+<div class=test4>
+<span>				<b>A</b></span>
+</div>
