A design review comes back saying the heading looks smaller than the mock. To answer that you open DevTools, hunt for the element in a tree forty divs deep, find the computed styles panel, read one number off it, and by then you've lost the thread of what you were checking in the first place.
DevTools is a fantastic tool for the people who live in it. It's also built for debugging, and debugging is a different job from looking. A debugger assumes you arrived with a hypothesis. Most people arrive with a question.
For everyone else - a designer checking whether that heading is really 32px, a writer counting characters, someone measuring a gap - it's a lot of panel to open for one small answer.
One click, five answers
Web Content Inspector is that small answer as a button. One click on any page and it surfaces text properties, media dimensions, layout structure, spacing and the visual boundaries of elements - the things you'd otherwise go spelunking in the console for.
The boundaries do more work than the rest of it combined. Almost every layout argument is secretly an argument about where an element ends, and padding is invisible until something draws it for you. Two people can look at the same gap and disagree for ten minutes about whether it's a margin problem or a font problem, and neither of them is wrong, because you genuinely cannot see it.
The image dimensions are the other one I use constantly. I shoot the photographs that go on the sites I build, so I'm on both sides of every conversation about whether that hero is being served at three times the size anybody will ever see it at. It's usually a yes.
Why an extension and not a web tool
The obvious cheaper build is a site you paste a URL into. It doesn't work, and the reason is worth knowing if you're ever tempted: a tool that fetches the page gives you its render of it, not yours. Not your logged-in state, not your viewport, not the variant you were actually served, not the page after the third-party script finished rearranging it. The questions people have are about the page in front of them.
An extension is also just the shape that survives being useful. One click, from wherever you already are, on any site, no setup, no tab switch, nothing to remember. A tool with a setup step gets used twice.
It does come with the awkward constraint of the category: it has to run inside pages written by other people, with their own styles, their own stacking-order wars and their own ideas about what fixed positioning is for. An overlay that nudges the layout it's measuring is worse than no tool at all, because now you're debugging the thing you brought with you.
Built because I kept wanting it
I built it because I kept wanting it, which is the only good reason to build a tool. The ones built to fill a gap in a market tend to arrive with every feature and no opinion, because nobody involved has ever been annoyed by the problem.
This one has an opinion, and the opinion is that it reads the page and doesn't touch it. Inspecting stops being casual the moment the thing doing the inspecting can also change something. You start being careful, you start wondering whether what you're looking at is still what was there before you clicked, and casual was the entire point of it.
It's on the Chrome Web Store, it's been up since 2024, and it turns out I wasn't the only one who wanted the button.