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Even assuming - and this is a big assumption - that things go HTML 5's way with new browsers supporting it and developers using it, its Flash-killer days are still somewhat distant. HTML 5 still has to contend with the perpetual elephant in the room - legacy browsers.

It seems a forgone conclusion at this point that in the next few years HTML 5 will eventually be adopted and supported by all the major browsers, but what about today's browsers?

The Microsoft factor??...

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ORIGINAL: Tailslide
Here's a start: http://www.alistapart.com/articles/get-ready-for-html-5/

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HTML 4.0 (the markup language we all know and love) is based on a “rulebook” called SGML. In the SGML rulebook, element names are not case sensitive, you can have elements with optional closing tags (like <p>), and you can have attribute values without quotation marks. XHTML 1.0 and 1.1 are based on a rulebook called XML. In the XML rulebook, element and attribute names are case sensitive, every opening tag must have a closing tag, and attribute values must be quoted.

HTML 5 defines a markup language that isn’t based on either rulebook, but that can be written in either “HTML form” (or serialization, as the spec calls it) or “XHTML form.”

When you write markup in HTML form, you are allowed to leave off some closing (and opening!) tags, you don't have to quote attribute values if they don’t contain blanks, and element and attribute names aren’t case sensitive. You also get to use some of the XML markup; you can have a trailing slash on elements like <img /> or <hr />.

When you use the XHTML serialization, you have to follow the XML rules mentioned a few paragraphs ago.