HTML5 is widely viewed as a watershed technology that makes it easier to use multimedia and construct applications that run in the browser. These benefits hinge on the widespread expectation that complex applications increasingly will run inside the browser or the browser will function as the GUI to cloud-hosted apps. If you've used Google Docs and Zoho's online spreadsheets and word processors, then you'll understand how this scenario works.
HTML5 changes this by specifying a range of capabilities that the browser must support, including video, drawing, text manipulation, and offline storage. The applications then simply invoke them via commands, rather than sending bits over the network to run on the client or using plug-ins.
While interest in HTML5 has been growing, Microsoft's slow adoption of the spec has been a barrier to jumping in fully. Because IE is the dominant browser in the market, any HTML5 development would have an indeterminate outcome if it couldn't be tested in the Microsoft browser. Before the release of IE9, the rendering engine in IE--known as Trident--didn't fully support several standards essential to HTML5, including several HTML5 syntactical advances. The new release of Trident, version 5.0, can render much of HTML5, as well as Cascading Style Sheets 3.0 and Scalar Vector Graphics (SVG).
SVG uses XML to specify a drawing or text. It relies entirely on the browser to take the data elements and render them as lines, curves, colors, and shapes. For years, IE has been the only browser without native SVG support. This situation accounts for the long delay in adoption of SVG as a common format for specifying 2-D graphics and illustrations. The possibility that Microsoft might similarly fail to adopt core elements of HTML5 was a concern that couldn't be ignored until the Redmond giant released an official browser version with these capabilities.
IE9 gives both HTML5 and SVG a shot in the arm. Perhaps sensing the importance of the release to developers, Microsoft added other tools that expand previous capabilities to let developers go behind the scenes and explore the execution of a Web page, by viewing and stepping through the code and even profiling its activity.
As Web applications become a larger part of workers' daily lives, it appears that Microsoft's browser will remain at the forefront--no longer just based on market share, but also because of its standards support and the technology with which it equips developers.