# Get the attributed value of 'prop' or null if 'prop' is undifened
	#
	#     var img = new HTMLTag("img")
	#     img.attr("src", "./image.png").attr("alt", "image")
	#     assert img.get_attr("src")     == "./image.png"
	fun get_attr(key: String): nullable String do
		if not attrs.has_key(key) then return null
		return attrs[key]
	end