# Get CSS value for 'prop'
	#
	#     var img = new HTMLTag("img")
	#     img.css("border", "2px solid black").css("position", "absolute")
	#     assert img.get_css("border")    == "2px solid black"
	#     assert img.get_css("color")     == null
	fun get_css(prop: String): nullable String do
		if not css_props.has_key(prop) then return null
		return css_props[prop]
	end