There was a lot of buzz this year at SXSWi about HTML5 and CSS3. HTML, HyperText Markup Language, has been the standard language of the Web since its beginning. Versions of HTML4 have been used since 1997, so it's been a long time since we've gotten a major update to basic web language. CSS, Cascading Style Sheets, allows developers to style elements on a web page, making pages standard and friendly for viewers. With the innovations of HTML5 and CSS3, developers can make websites faster to run, easier to develop and nicer to view.
HTML5 has several new elements for developers to use including semantic replacements for common elements on a page like headers, navigation and footer blocks. Gaining the most steam was HTML5's support for <video> and <audio> tags. When developers want video and audio to play on a website, they almost always turn to Adobe Flash Player. HTML5 could eliminate the need for Flash in some circumstances, allowing pages to perform faster and still look and interact the same way.
CSS3 has a whole host of new ways to style web pages including adding web fonts, rounded corners, shadows, shading, and much more. These attributes mean developers will be less reliant on images, decreasing the amount of time it takes for a page to load.
As is often the case in web development, cool new technologies take some time to be implemented. Currently, HTML5 and CSS3 have the support of four of the five major browsers, the exception being Internet Explorer. Expected to be released in 2011, Internet Explorer 9 will support HTML5 and CSS3, however, until large numbers of consumers upgrade to IE9 or other modern browsers, it'll be awhile before a lot of these techniques can be used.
Here's a great page for visualizing how browsers are supporting HTML5 and CSS3. You'll instantly notice how current Internet Explorer options don't measure up.