During the 1990's, the HTML standard had been evolving pretty steadily. But as Internet Explorer, with it's non-standard extensions, gained popularity with developers it caused a divergence between the standard and the implementation, which resulted in the evolution of the markup language becoming stagnant for almost a decade. Attempt to make some additional improvements in HTML, as xHTML, unfortunately backfired and resulted in further chaos in HTML development.
Finally, HTML version 5 promises to bring order back to the chaos. There is enough support from vendors and standards bodies behind HTML 5 that developers who have been creating web sites using HTML and xHTML as well as Flash can confidently start their migration to this latest version of HTML. Many web browsers, including Chrome, Firefox, Opera and Safari already support many of the features of HTML 5. In particular, developers who are concerned about their web site content and apps running on mobile devices should start their migration to HTML 5.