Update web-platform-tests to revision b'468d01bbd84da2babf265c6af46947be68713440'

tests/wpt/web-platform-tests/css/css-pseudo/README.md

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+## Highlight pseudos
+
+There are many subtle browser differences in rendering these pseudo-elements,
+most of which comply with the specs, so here are some hints.
+
+Use the helpers in `support/highlights.css` and `support/selections.js` where
+possible. They include rules and functions for “basic” but tricky tasks like
+setting up a test area, selecting content, and triggering spellcheck.
+
+When creating complex layered references, start by wrapping your text in a
+relative container, then prepend absolute layers with copies of that text, then
+mark up those copies with spans. Make everything transparent initially, and set
+visible styles on the spans only. The absolute layers will perfectly overlap
+your original text, which you can keep for external layout. For example:
+
+```html
+<div class="container">
+    <div class="spelling-error"><span>Teh</span> <span>dgo</span> and
+        <span>teh</span> <span>sphixn</span>.</div>
+    <div class="selection">Teh d<span>go and te</span>h sphixn.</div>
+    Teh dgo and teh sphixn.
+</div>
+```
+```css
+.container { position: relative; color: transparent; }
+.container > * { position: absolute; }
+.spelling-error > span { background: ...; color: ...; }
+.selection > span { background: ...; text-shadow: ...; }
+```
+
+Simplify this pattern at your own peril! For example, if you set backgrounds
+directly on layers as your highlight backgrounds, they will always be exactly
+`line-height` tall, but even if your `line-height` is 1, the actual line boxes
+and so on can still be taller (unless they contain Ahem text only).