Paste your page HTML and measure how it really behaves on a phone: missing viewport tag, sideways scrolling, tiny text, small tap targets, oversized images.
The test renders your HTML in a sandboxed frame at the selected phone width and measures the real layout: page width, overflowing elements, font sizes, tap targets and images. It does not estimate. It cannot fetch a URL, because browsers block reading other websites — paste the source of your page instead. JavaScript inside the pasted page is not executed, so pages that build their layout with JavaScript are measured as their static markup.
This test answers one question: does this markup survive a phone screen? You paste the HTML source of a page, pick a phone width (320, 375 or 414 px), and the tool renders that markup in a sandboxed frame at exactly that width and measures the real layout with the browser's own layout engine. It reports the things that actually break on mobile: a missing or fixed-width viewport tag, content that is wider than the screen and therefore scrolls sideways, hard-coded pixel widths, text below 12 px, form fields below 16 px font size (iOS zooms in on focus), tap targets under 44 by 44 px, and images wider than the screen. Two honest limits: the tool cannot fetch a URL, because browsers block reading other websites, so you paste the source; and JavaScript inside the pasted page is not executed, so a layout that is built entirely by JavaScript is measured as its static markup.