Performance. It is faster for us not to have to check for a style called
'width' if we don't have to.
Isn't it quite a high price to pay for losing the ability to write fully semantic code? In many situations it becomes impossible to separate position and dimension declarations from the tags representing the components. Ideally shouldn't we be able to perform all styling and layout from CSS as per HTML?
Is this something that is set in stone, or is it something that might change at some point? Most CSS styles are set once at startup, so performance wouldn't be too much of a problem in a lot of cases.
There are no plans to change it at this time. Runtime performance including
when things are animating is still pretty important, especially as we look
to provide Flex on mobile devices.