I've used browser plugins for this for a decade already, for Firefox even before Chrome was written. I guess a bookmarklet is more accessible and easier to install, though. That said, if you really want to break new ground, The Android layout editor recently gained the ability to show multiple layouts of different size/orientation at once. It would be nice if someone made a browser equivalent. I.e. so you can easily tweak some HTML/CSS and see several different windows with different resolutions update instantly, preferably not having to go click refresh on all of them. Although monitoring file changes may require going back to more powerful technology, like plugins.