I found that there is no capture for @charset in CSS.tmLanguage.
I fixed that temporarily for myself by changing line 155 into <string>^\s*((@)(?:import|charset)\b)</string>
Also, the scopes punctuation.definition.comment.html.css and comment.block.html.css are missing (similar to punctuation.definition.comment.html.js and comment.block.html.js in JavaScript.tmLanguage).
I added them below line 448 of my CSS.tmLanguage file:
<dict>
<key>captures</key>
<dict>
<key>0</key>
<dict>
<key>name</key>
<string>punctuation.definition.comment.html.css</string>
</dict>
<key>2</key>
<dict>
<key>name</key>
<string>punctuation.definition.comment.html.css</string>
</dict>
</dict>
<key>match</key>
<string>(<!--|-->)</string>
<key>name</key>
<string>comment.block.html.css</string>
</dict>
Ko3