# Read a XML comment.
	# Used by `check_html`.
	private fun read_xml_comment(first_line: MDLine, start: Int): Int do
		var line: nullable MDLine = first_line
		if start + 3 < line.as(not null).value.length then
			if line.as(not null).value[2] == '-' and line.as(not null).value[3] == '-' then
				var pos = start + 4
				while line != null do
					while pos < line.value.length and line.value[pos] != '-' do
						pos += 1
					end
					if pos == line.value.length then
						line = line.next
						pos = 0
					else
						if pos + 2 < line.value.length then
							if line.value[pos + 1] == '-' and line.value[pos + 2] == '>' then
								first_line.xml_end_line = line
								return pos + 3
							end
						end
						pos += 1
					end
				end
			end
		end
		return -1
	end