Not that I'm aware of. You normally write it directly into HTML code.
Thanks for the reply Jon.
This is a huge part of professional photographers work these days as digital content providers, to correctly prepare images with correct names and alt tags.
Surely with all the sophistications of LR, there's a way?
I think it really depends where images are to be hosted. For example there is a plug-in for Wordpress blogs that can upload from LR and turn LR metadata fields such as Title, into Alt attributes that correspond with the WP interface. I've never used it but I'm sure you could search for similar LR plug-in's but I guess every client will have different needs.