html.HTMLElement: An add_style method

[nikochiko]

An add_style method would be nice to have and much more preferable over the traditional way of modifying the style attribute as a string.

There are two ways this could work:

el = html.div()
el.add_style("width", "100%")

or with keyword arguments,

el = html.div()
el.add_style(width="100%")

The second one is easier to type but might get problematic when dealing with keyword names that are not representable as Python variable names.