It's the "ht" in http & html
[DOC] [TLE] Hypertext Transfer Protocol -- HTTP/1.0 - W3C [PAR] A proxy is a forwarding agent, receiving requests for a URI in its absolute form, 
rewriting .... SP | HT Comments may be included in some HTTP header fields by 
..... most HTTP/0.9 servers are limited to responses of type "text/html" and 
therefore... [DOC] [TLE] HTTP/1.1: HTTP Message [PAR] HTTP messages consist of requests from client to server and responses from 
server ... over multiple lines by preceding each extra line with at least one SP or 
HT.