The RFC for SRV records specifies that it may not be used by pre-existing protocols which did not already specify the use of SRV records in their specifications. I.e. no SRV in the HTTP spec – browsers are, by the SRV standard, prohibited from using it.
This does not prohibit a new HTTP 1.2 standard from specifying the use of SRV records, though. However, Mark Andrews proposed this in April 2007 to the IETF HTTP working group, but got no response.