It would be nice to add a bit more of an explanation for some of these things. eg:
- There's a bold warning about float elements with display: block set - "Do not set both". Why is this so important?
- "Skipping the doctype can cause issues with malformed tables, inputs, and more" - some examples of what could go wrong would be really handy here. Or a link to a more in-depth explanation?
In general some more links would be great.
One small example that I've seen go wrong, elements don't center with CSS if you skip the doctype. Seems to ignore margin-left and margin-right.
Omitting the doctype basically sends the browser into "quirks mode" and then the page is rendered with old browser rendering bugs.
It's a bad scene. Avoid it at all costs.