CSS Pseudo 4 specifies that the background of highlight overlays is drawn over any text shadows of layers below it. That means, it should also obscure any shadows applying to their originating element.

Gecko currently doesn't follow the specification here.

Simple example:

data:text/html,<p>Text</p><style>p{font-size:10em;text-shadow:0 0 20px white, 0 0 20px white;}</style>

When you select the text, the text shadow should not be visible anymore.

Once this works according to the spec., the UA style added in bug 1247177 is not necessary anymore.