Adopts mdo's style guide for CSS (and HTML)

[jzaefferer]

This style guide for CSS looks pretty good to me: http://mdo.github.io/code-guide/#css

While that is online and actively maintained, we could just link to it. Its MIT licensed, so we could also integrate it here.

From the HTML section we could take over the sections on "Attribute order" and "Boolean attributes", if we want to adopt those.

[scottgonzalez]

There are multiple differences between our CSS style and mdo's CSS style.

• We never use soft tabs.
• We don't change spacing rules for commas based on location.
• We don't use spaces to line up multiple rules.
• We don't collapse single property rules to one line.
• We're planning on switching to BEM so class naming is very different.

[scottgonzalez]

For HTML, I think we already follow the boolean attributes rule, though we do have some files that need to be valid XHTML, so it's not possible there.

I disagree with the attribute ordering. I think type should always be first for <input>.

[jzaefferer]

Not much left over then. I'll leave it to @uGoMobi to write a style guide for CSS, if he's interested.