How can I outline the elements of an HTML page in the browser to see the layout of the boxes? Modify the user agent style sheet?

In the Chrome devtools, Firefox devtools, Safari, Opera, etc., if I inspect an element I can see its bounding box nicely outlined when I mouse over the code for that element in the source panel. That's great. But what if I'd like to see how all (or most of) the elements on the page are laid out? For instance, maybe I'd like to see something like this:

In the Firefox "Style Editor" I've added these styles:

div { border: 1px dotted pink }
p   { border: 1px solid green }
a   { border: 2px solid yellow }
li  { border: 1px dashed cyan }
img { border: 1px solid purple}

(Chrome can't do this since it doesn't support the UAAG 2.0 web standard for accessibility). Since the user agent style sheet overrides the styles from the page, I see the kind of outlines I'm looking for.

Now this is just a hack, and perhaps is sufficient, but are there other tools that do this, or something in the devtools that I didn't find?

Note: I did find this answer regarding the "Show composited layer borders" under the Rendering menu option in Chrome, but it's not really what I'm looking for:

https://superuser.com/questions/774424/grid-overlay-showing-up-as-soon-a-i-launch-chrome-developer-tools

It turns out I was looking for a browser extension a friend had mentioned a long time ago: the "Web Developer" extension.

http://chrispederick.com/work/web-developer/

Here's what it looks like outlining the block level elements:

It's available for Chrome, Firefox, and Opera. Apparently not for Safari.