GridSim drops a column (x) and baseline (y) grid onto any live website, so you can see what lines up and what drifts. The best part: a bookmarklet (Safari and Chrome) you drag to your bar once, then click on any site to overlay the grid in-page, with live column and gutter controls. You can also paste a URL, check it in a real iPhone or Android frame, compare two devices, and take the grid into Figma as an overlay or grab the CSS. Free, no sign-up.