CSS Nested Comments

CSS does not have a nestable comment syntax.

You could instead wrap the rules in something which does nest, but will not match anything, such as a non-existent media type:

@media DISABLED {
    #container {
        width: 90%;     /* nested comment here */
        margin: 0 auto;
    }

    #container .class {
        width: 25%;
    }
}

This does have the caveat that it will not be obviously a comment (so it will not be removed by tools that remove comments), and will cause the browser to spend time on parsing it and finding that it doesn’t match. However, neither of these should be a problem for temporary development use, which is the usual reason one would want to comment out a large chunk simply.