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>(&lt;!--|--&gt;)</string> <key>name</key> <string>comment.block.html.css</string> </dict>

Ko3