Two names often seen as competitive technologies, but this is not my perspective!
If you look at it, it still is the old problem of richness versus reach.
Silverlight is able to provide stunning UI experiences over the web (nowadays also known as Cloud), while HTML 5 will be the future standard of anything that spells “browser”.
Fair enough one might think, but this is not all. Due to the fact that Silverlight is able to plug into HTML and HTML can be rendered in a Silverlight control there are quite some interesting possibilities for bidirectional integration.
And even better news are that Microsoft is emphasizing to be supportive for both of them.
This enables developers to choose richness where it is appropriate and reach where it is required. Both without running into strangulating technical limitations.
The only thing I would wish for is that both technologies find their way down to embedded device development e.g. with Windows CE pretty soon, too.
They are far to valuable to reserve them for Windows Phone 7 only.
Just my 2 cents (Euro cent ,of course )