Checking a layout at the right size usually means dragging a window edge and guessing. Web Screen Ruler replaces the guessing with presets: snap the browser to common phone, tablet, and desktop dimensions, or type an exact width and height.
On top of any page you can drop pixel rulers and a configurable column grid, so you can measure spacing and confirm alignment against the design rather than eyeballing it.
For pixel-perfect work, overlay a reference image directly over the live page and dial its opacity up and down. Line the real thing up against the mockup, spot the drift, and fix it — all without leaving the tab.