Not some companies. All companies, including Google for Gmail, Yahoo for Yahoo mail, Microsoft for Outlook, etc have very limited support for HTML in their HTML rendering engines.

Instead of using floats and margins, you have to revert to using <table> for layouts. Otherwise, your emails just won't render correctly, much of the time.

It's not possible to support HTML fully in Webmail clients, either. Even without scripting, you could write an HTML email that overlays a malicious button on top of Gmail's web interface itself, and it would be too easy to phish people.