1. manipulate the browser history and build awesome asynchronous user interfaces like this one or highly responsive and intuitive interfaces like this one
2. know the user location and serve him customized content (remember that ‘date single women near [your location]’ ads? :P)
3. store data in the user browser: more than 4k of data, stored on the client, that persists beyond a page refresh and isn’t transmitted to the server. For example to store the number of the last page read in an online book or to save the settings of an online game between browser restarts
4. use new input types (like the slider, color picker, date picker, etc) and unload the validation task on the browser
5. allow the user to take your app offline, edit it, and synchronize it when back online. Example: GMail offline
6. Play video for everybody
7. Use the canvas element to draw amazing 3D text or sticky things (try this link on your mobile device, and shake it :))

Yes, you can use all of these features plus many, many more right now. Most of them will gracefully degrade if the browser is too old or simply does not support them.
Now read the list above once again thinking about the implications on the mobile browsing.

Here are some interesting things I found from Dive into the HTML5 ebook:
Note: a couple of months ago, when I started reading this ebook, it was available under diveintohtml5.org. However, the author – Mark Pilgrim – is 410 Gone from web. However, his ‘Dive into …’ book were mirrored and they are still available.

1. XHTML vs HTML parser

Even if you declare your DOCTYPE as being XHTML, if you serve the page as Content-Type: text/html, it will be parsed by the HTML engine, not by the ‘draconic’ XHTML one. #

2. Double negative to convert to Boolean #

/* You can use the double-negative trick 
to force the result to a Boolean value (true or false).*/
return !!document.createElement('canvas').getContext;

3. Skip ‘skip to content’ 🙂

No need to add ‘skip to content’ links at the beginning of your page if you are using HTML5’s nav element. The screen readers will handle it correctly. [#]

4. Character encoding

Web Server’s HTTP Header:

Content-Type: text/html; charset="utf-8"

Briefly, this says that the web server thinks it’s sending you an HTML document, and that it thinks the document uses the UTF-8 character encoding.

<meta charset="utf-8" />
or
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

Briefly, this says that the web author thinks they have authored an HTML document using the UTF-8 character encoding.

Both of these techniques still work in HTML5. The HTTP header is the preferred method, and it overrides the meta tag if present. But not everyone can set HTTP headers, so the meta tag is still around. [#]

5. Document outline

You can have multiple h1 tags in the same page. [#]
You can check your HTML5 page outline using this outliner.

6. Use Modernizr

Modernizr is a small JavaScript library that detects the availability of native implementations for next-generation web technologies, i.e. features that stem from the HTML5 and CSS3 specifications. Many of these features are already implemented in at least one major browser (most of them in two or more), and what Modernizr does is, very simply, tell you whether the current browser has this feature natively implemented or not.

if (Modernizr.canvas) 
if (Modernizr.canvastext) 
if (Modernizr.video) 
if (Modernizr.localstorage) 
if (Modernizr.webworkers) 
if (Modernizr.applicationcache) 
if (Modernizr.geolocation) 
if (Modernizr.inputtypes.date)
if (Modernizr.input.placeholder) 
if (Modernizr.input.autofocus) 
if (Modernizr.history) 

//Example video:
if (Modernizr.video) {
  // let's play some video! but what kind?
  if (Modernizr.video.webm) {
    // try WebM
  } else if (Modernizr.video.ogg) {
    // try Ogg Theora + Vorbis in an Ogg container
  } else if (Modernizr.video.h264){
    // try H.264 video + AAC audio in an MP4 container
  }
}

More on the video element

7. HTML5 canvas element

It kind of looks like Flex without the Flash Builder 🙂 To use it in older IE browsers as well, use ExploreCanvas [#]
Here’s a nice application which lets you draw 3D sketches with animating strokes on HTML5 canvas.

1. Test if the browser supports navigation:

if (navigator.geolocation) {  
  /* geolocation available */  
} else {  
  /* geolocation not supported by this browser */  
} 
//---or use Modernizr
 if (Modernizr.geolocation) {
...
//---or be geek
if(!!navigator.geolocation){
...

2. Getting the current position: navigator.geolocation.getCurrentPosition()

The getCurrentPosition() method initiates an asynchronous request to detect the user’s position, and queries the positioning hardware to get up-to-date information. When the position is determined, a specified callback routine is executed. You can optionally provide a second callback to be executed if an error occurs. A third, optional, parameter is an options interface where you can set the maximum age of the position returned and the time to wait for a request. [#]

navigator.geolocation.getCurrentPosition( 
		geo_success, 
		geo_error, 
		{enableHighAccuracy:true, timeout:30000, maximumAge:60000 }
	);

3. Geolocation success callback function

The success callback function will be called with a single parameter, an object with two properties: coords and timestamp.
(more…)