CC: Make HTML/CSS readers (especially HTML embedded in MD) way more forgiving

[vxlcoder]

the current HTML/CSS readers expects perfect syntax. this isn't always necessary. make the readers way more forgiving, and provide helpful warnings to the console.

right now, if syntax isn't correct (ex: closing a self-closing tag like </input>), the reader throws an exception! this is especially bad in UI updates, as it crashes RF and causes Blender to get stuck in a nasty, unrecoverable loop.