From microsoft/vscode#35300

Try the following code sample in Atom or VSCode:

@charset "UTF-8";
@keyframes fadeIn {
}
@page :first {
    margin: 1in;
}
@supports (display:flex) {
}
@namespace url(jttp://www.w3.org/1999/xhtml);
@import 'ddd'

Note that
@charset, @keyframes, @page, @support, @namespace
are scoped as variables.
-> Should be a keyword, e.g. keyword.control.at-rule.less