[fysiks]

Since the MOTD is implemented by the game client, you can't change what the MOTD is capable of rendering. Like jimaway said, Half-Life (the original game) doesn't support HTML-based MOTDs. Counter-Strike 1.6 and Day of Defeat (and some HL1 mods) have a basic web browser in the MOTD window which allows them to render basic HTML, use CSS, and execute Javascript.