Great concept. I use a tiling window manager, too, so this solves a lot of issues of having to readjust scroll positions all the time.

However, I think it should have a global minimum and maximum zoom percentage that can be set by the user rather than a pixel width. When a window gets too small or too big, some pages become hard to read or insanely zoomed in. Using a percentage instead of absolute pixel values also plays nicely with non-standard DPIs.