Obsolete HTML features
If you have been writing HTML for a few years, you might be surprised to learn there are parts of HTML that are now considered obsolete. It's important to be aware that there are two camps of obsolete features in HTML: conforming and non-conforming. Conforming features will still work but will generate warnings in validators. Realistically, avoid them if you can but they aren't going to make the sky fall down if you do use them. Non-conforming features may still render in certain browsers but they may not. It's certainly not guaranteed.
In terms of obsolete and non-conforming features, there is quite a raft. I'll confess that there are many I have never used, and some I've never even seen! It's possible you may experience a similar reaction. However, if you're curious, you can find the full list of obsolete and non-conforming features at http://www.w3.org/TR/html5/obsolete.html. Notable obsolete and non-conforming features that you may occasionally encounter are strike, center, font, acronym, frame, and frameset.
There are also features that were present in earlier drafts of HTML5 that have now been dropped. hgroup is one such example. The hgroup element was originally proposed to wrap groups of headings: an h1 for a title and an h2 for a subtitle might have been wrapped in an hgroup element. However, discussion on the hgroup element is now frustraneous as it's now gone the way of the Raphus cucullatus.
