Website Meta Language is a sophisticated GPLed offline HTML authoring tool. Its homepage contains a lot of information about it.
WML (not to be confused with WAP Markup Language) used to be actively maintained by Ralf S. Engelschall and Denis Barbier. For a long time (since about 2001) it had been unmaintained, but recently Shlomi Fish started revamping the site, making sure it can be built on cygwin again, and also released the Latemp framework on top of it. (After factoring out most common functionality from the sites he built using Website Meta Language).