Extend CSS tree shaking to remove selectors for non-existent IDs and elements

[westonruter]

If a CSS selector contains a reference to an ID, like #subscribe-email input { ... } and there is no element with the ID of subscribe-email in the document, then this CSS rule should be removed. Additionally, if a CSS selector includes a reference to a tag name for an element that doesn't exist in the document, then this CSS selector should also be removed from the document.

On Twenty Fifteen without these changes, the CSS is 42421 bytes. With this branch, the CSS is 35114 bytes. So about 20% smaller.

This also removes trailing semicolons from rules to save additional bytes.

[kienstra]

Approved

Hi @westonruter,
Thanks, this looks good. It really helps to remove rules where the selector has an ID or tag that's not in the document.

There were similar decreases in CSS size when testing with Twenty Sixteen in Paired Mode. Before, there were 37005 bytes of CSS, and with this PR there were 30747 bytes.