Steps to reproduce
1. Take any supplementary Unicode character, e.g. tetragram for centre (U+1D306)
2. Break it up in surrogate pairs and append a space, e.g. `\d834\df06 `
3. Use this non-standard escape sequence in CSS
In CSS, `\d834\df06 ` should be considered as two separate character escape sequences. Since these are two surrogate halves, two replacement characters (U+FFFD) should be shown.
The surrogate halves are grouped into a single glyph. This shouldn’t happen. See http://lists.w3.org/Archives/Public/www-style/2012Feb/0006.html.