# Set a CSS 'value' for 'prop'
	#
	#     var img = new HTMLTag("img")
	#     img.css("border", "2px solid black").css("position", "absolute")
	#     assert img.write_to_string      == """<img style="border: 2px solid black; position: absolute"/>"""
	fun css(prop: String, value: String): HTMLTag do
		css_props[prop] = value
		return self
	end