You can <a href="url" class=action>turn on the lights</a>.
However, since an action is contained to the environment you are
within, this approach would also require the addition of a new URL
scheme (perhaps "local:turn on lights").
But if you're inventing your own URL scheme anyway, there's no need for
the new attribute on the <a> tag --- the use of the new URL scheme itself
should be enough of a cue for your client software to invoke whatever
special-case actions you have in mind, e.g.,
You can <a href="mud_environment:turn_on_lights">turn on the lights</a>.
This involves no changes whatever to HTML itself, since the grammar of
URLs is not part of the HTML spec.