Determine exact wording of "You are in edit mode." message when edit mode is enabled.

Maybe for the message we can get away with just:
Click a page element to edit it.

The bit about menus and forms being disabled to me sounds like we're explaining that we thought about how this should work. Well, well done us but no need to communicate that to the user :)

Aside: I'm not sure about expressing state in the label of the Edit button though. Maybe the button should say "Exit edit" when in edit mode instead of "Editing".

My suggestion is

"Click on highlighted area to edit. Leave edit mode to go to a different page."

I like the suggestion from #4, which is short and simple.

A couple of suggestions I'm skeptical of, for accessibility...

#7: "Click on the pencils for more options." Can a screen reader user find these "pencils"? Think about what the accessible (text) name of the button is, because that is what a screen reader will announce when the pencil is focused.

#2 and #5: "Click on any highlighted area to edit". I'm wondering how a screen reader user identifies the highlighted areas. Maybe it comes from the accessible name of the pencil/buttons. This doesn't work so well IMO in the buttons we currently have from contextual.module, where every button/menu has the repeated phrase "open configuration options", "configure block". I think we could make these names a bit more specific without adding verbosity, e.g. a button called "edit title block".

Re: 10 - that's an interesting judgement call. I'd say "last resort" rather than outright bad practice. Not very elegant - it's going down the road of alternative accommodations, rather than truly inclusive design.

Having said that, much accessibility comes down to what we can get away with. In Drupal core, the toolbar admin UI in CKEditor module has quite a lot of help in Drupal.announce(), so we've taken that approach before.

This issue is the sort of thing that would benefit from tests with real AT users, but of course we've got to build something before we can get that kind of feedback. We can refine it in future releases.

An idea: keep the "now in edit mode" message short and simple, and consider using aria-describedby for information about individual buttons and links. This would be persistent text that can be found when exploring a UI, and respect the verbosity settings of some screen readers. Drupal.announce() messages are only spoken once, so a user might miss them. (In fact, they're not guaranteed to be spoken at all.)

The forthcoming aria-roledescriptionproperty in ARIA 1.1 might turn out to be useful, but we're waiting on browser and AT support. It would supplement the name of a button, unlike aria-label which would override it.

Another idea. Instead of saying "click on the pencils", we could have the message say "click on the edit buttons" and include the pencil icon in the message tooltip as a bonus clue for sighted users.

An admin theme in contrib could replace pencils with another icon, without having to change the message text or accessible name.

We only need to discuss the pencils bit if we think it's useful to add it :)

An idea: keep the "now in edit mode" message short and simple, and consider using aria-describedby for information about individual buttons and links. This would be persistent text that can be found when exploring a UI, and respect the verbosity settings of some screen readers.

This sounds like the actually accessible design approach. @andrewmacpherson something to break out into it's own issue I think?

Are we good with "Click a page element to edit it." then?