It's actually a web frontend for a CLI application:
> Also, I would prefer this as a CLI application, not as a webpage.
curl http://wttr.in/london
Using curl removes the HTML tags? Nice feature (though a little obscure).
I think what they're doing is using the user agent header like the accept header, which makes sense for that specific idea, as the browser version is actually just kind of imitating a console. It currently doesn't work because they ran out of queries on their backend service, but I think they were not stripping html, but delivering a terminal text version with color escape characters.
What is that cruft at the bottom of the page?
Also, I would prefer this as a CLI application, not as a webpage. Further it should have flags that make its output more friendly to further processing (e.g. grep).
Otherwise, nicely done.