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Kinda cool, but it doesn't work well if you have a widescreen monitor with the browser full screen & your site is center aligned.

It seems to only work if you left align all your content.

You can work around this by resizing your browser to the width you want to test at.

Right, but at that point you're just doing all the work a browser extension can do for you.

Not really, you just need to resize it to the minimum width of your site. It's easy to see.

Presumably you (the developer) have a screen much wider than your site.

This should still give you useful data about the vertical placement of elements on your page, no?