MXP

Developed in the late 1990s by Zugg Software, the outfit behind the zMUD client. The server switches it on through Telnet option negotiation (option 91) and can then send tags such as <send>, which makes an object's name clickable, or <color> and custom entities. Clients without support just get plain text.

Part of the inspiration came from Pueblo, which had tried to embed real HTML. MXP settled for a narrower subset and won wider support as a result, but the spec was never especially rigorous and implementations disagree. Newer games prefer to push structured data over GMCP and let the client do the drawing.