The three levels of HTML5 usage
mathiasbynens.be
mathiasbynens.be
Second, the article is unclear on the "optional" nature of the solidus.
I'm pretty sure that the use of the solidus depends on whether you are using an HTML or XHTML serialization.
If you are sending the page as content-type text/html, you need to follow HTML format and leave out the solidus (that is, you follow the same rules as you would for HTML4).
If you are using the XHTML serialization you need to be using XML syntax and send the page as application/xml or application/xhtml+xml.
In the XHTML serialization of HTML5, the solidus isn’t required either; you can choose to close the elements instead.
The following examples are both confirming/valid HTML5 in ‘XML mode’ (they’re valid XHTML1 as well):
a) With solidus
<img src="foo" alt="bar" />
<img src="foo" alt="bar"></img>
Some additional info here: http://dev.w3.org/html5/spec/syntax.html#void-elements
I'm skeptical about the increased casual nature of what makes for an acceptable HTML-serialized HTML5 doc. Seems it just makes it that much harder to write a robust parser and tools. Especially the continued use of empty attribute syntax, which is just nasty. I understand it's to allow plain old HTML docs to be acceptable, but still ...