CSS Pseudo 4 specifies that the cascade needs to be taken into account when applying styles to highlight pseudo-elements.

Furthermore, the spec. explicitly notes that authors should use :root::selection to apply a document-wide selection style.

Gecko doesn't seem to follow the specification in this case, as styles defined in :root::selection don't have any effect.

Example:

data:text/html,<p>Text</p><style>p{font-size:10em;}:root::selection{background-color:black;color:yellow;}</style>

According to the spec., when you select the text, the specified background and foreground colors should be applied.

Once this works according to the spec., the UA style added in bug 1247177 may be changed to use :root::selection.

Sebastian