# Set a 'value' for 'key'
	#
	#     var img = new HTMLTag("img")
	#     img.attr("src", "./image.png").attr("alt", "image")
	#     assert img.write_to_string      == """<img src=".&#47;image.png" alt="image"/>"""
	fun attr(key: String, value: String): HTMLTag do
		attrs[key] = value
		return self
	end