Use HTML5 Now, Dammit!

or, at the very least use <!doctype html>

If you remember nothing else, remember this: <!doctype html>

Simplest DOCTYPE that will keep browsers out of dreaded quirks mode

For a great overview of three strategies for picking a doctype, read Jenn Lukas' article in Cognition

What is HTML5?

Simply put, it's “the latest iteration of the web’s lingua franca” — Jeremy Keith

There are many technologies which often get lumped under the banner of HTML5. Some make less sense than others (SVG, CSS3)

Just remember, HTML is markup

What isn't HTML5?

HTML5 may or may not be:

The silver bullet for all the web's problems
Unicorns
Rainbows

Why?

More semantic elements to choose from mean less cruft, more efficient markup

header
footer
article
section
nav

No more plug-ins, audio and video may now be displayed directly by the browser

App-like features: off-line storage, drag-and-drop, multithreading (via Web Workers)

New <input> types

email
tel
date
datetime

<audio/> & <video/>

Browser-native media, no plug-ins necessary

Codecs are a pain

IE is a pain, but no more than usual

Keep your eyes peeled for a detailed presentation on <video/> from Tim Badaczewski!

A Note on Progressive Enhancement

You can use nearly all features of HTML5, CSS3, and the latest JavaScript, but treat all of the above as bonuses for those fortunate enough to use modern browsers. Don't forget the poor folks shackled with Internet Explorer 6 (or 7, or 8...)

modernizr
html5 shim
polyfills – “A shim that mimics a future API providing fallback functionality to older browsers” — Paul Irish

Mobile

WebKit dominates in some form or another

HTML5 is good for mobile

More semantic elements means less classitis, fewer bytes
Offline storage
Web workers
Geolocation