If you add some kind of "snap to DOM" ability or whatever, so that the rulers align with "places" already "defined" by the DOM+CSS, it might even be better.
i actually have something like this already made (with guides and all) that i've been dragging my feet (no pun intended) to open source since it's a bit rough around the edges. i'll put it up in the next week or so. feel free to watch the repo in the likely case it'll not make it to the front page.
This brings back that sinking feeling when a client gets out the ruler and holds it up to their screen and then down to the printout the designer gave to them. shudder
Am I the only one who thinks the vertical lines should be triggered when you click the top ruler and vice versa? Now it's the other way around and it feels counterintuitive.
It's the way Photoshop (and the rest of Adobe's suite) has done it for ages and it's intuitive in the sense that you're creating something of a copy of that ruler and its alignment.
Photoshop has been using the drag&drop method for the last 15 years, so I've gotten used to it. I can see your reason behind wanting to click a point on the scale, but adjusting the guide would require you to drag it orthogonal again anyway.
My coworker was looking for something like this a few days ago, and we gave up on our search. This is awesome. He's a front-end developer and nearly cried tears of joy when I showed this to him. Thank you from one small dev shop!
Most of the time, adding an option (particularly toggled modes) is a design failure. A much better solution would be to have the ruler be fixed to the border, but scroll along its axis.
You're telling me it's a design failure to give a choice when you're not giving explanations about why fixed would be better. The word "better" isn't a reason.
But it may not, obviosly.