Uh, just reviewing your html quickly, yes, it's broken. Badly.
Now, as to whether or not that would affect it from showing during a non-Sandbox custom made gamemode, I'm not sure.
I've had no experience with Steam's new webkit (nor am I even sure Gmod is using yet, I think it is).
Should really check your server console for errors when players join or run "ulx motd" from client console or "!motd" from chat.
Check all three.