A user account is required in order to edit this wiki, but we've had to disable public user registrations due to spam.
To request an account, ask an autoconfirmed user on Chat (such as one of these permanent autoconfirmed members).
IDE
vim
vim natively ships with CSS, Javascript & HTML syntax highlighting provided the feature is switched on.
vim also has completion features documented at http://vimdoc.sourceforge.net/htmldoc/insert.html#ft-css-omni
https://github.com/kaihendry/vim-html5/ provides an example with a video tutorial of howto integrate with validator.nu, jshint, csslint et al.
Emacs
XHTML5 or polyglot documents
Emacs ships with nxml-mode, a major mode for editing XML which does on-the-fly validation with RELAX NG schema. You can plug fantasai and hsivonen's schema into nxml-mode. The necessary shim code is included in the hober/html5-el project on github.