Jan van Leyden
Adventurer
Though DDI didn't switch to using Silverlight until late 2010, right?
Development would have started somewhat earlier, don't you think?
Have a look at The HTML5 test - How well does your browser support HTML5?. Even the most standards conform browser only pass about 80% of the tests, with tablet browsers being worse still. A few years back, the results have been truly disastruous.
From the point of view two years back HTML5 wouldn't have been an option. Flash might have been an alternative to Silverlight, but at the cost of Apple fans being nerfed.
That isn't to say that WotC software development shouldn't explore other technology venues for D&DN.