A responsive checker shows one page at several screen widths. Online checkers work well on public pages that allow being framed, and they fail on three kinds of pages: localhost, pages behind a login, and sites that send a header forbidding embedding. When the frame is blank, the cause is almost always that last one, and you can confirm it in ten seconds. For those pages use a tool that runs inside your own browser: Chrome DevTools device mode or a mobile view extension.
Which responsive checker for which page
| Page you want to check | Online checker | What works instead |
|---|---|---|
| Public page that allows framing | Yes | Any online checker |
| Page that forbids framing | Blank frame or an error | DevTools device mode, a mobile view extension |
| Staging site behind a login | Usually the login screen | DevTools or an extension in a tab where you are signed in |
| localhost | No, the tool’s servers cannot reach it | DevTools or an extension; for a real phone see testing localhost on mobile |
Why an online checker shows a blank frame
Most online checkers load your page inside an iframe. A site can tell the browser not to render it in a frame, with the X-Frame-Options header or the frame-ancestors rule of its Content-Security-Policy. MDN describes X-Frame-Options: DENY as blocking the page in any frame, and SAMEORIGIN as allowing it only when every parent frame has the same origin as the page. Setting it in a <meta> tag has no effect, it works only as an HTTP header. No checker can override this, because the browser enforces it, not the tool.
Many large sites send one of these headers. When we looked on 28 October 2026, bbc.com answered with X-Frame-Options: SAMEORIGIN, so it cannot be shown in someone else’s frame. That is the site’s decision and has nothing to do with your layout.
Check in ten seconds whether a site allows framing
Run this in a terminal and look for the two headers:
curl -sI https://example.com | grep -iE "x-frame-options|content-security-policy"If you see X-Frame-Options: DENY or SAMEORIGIN, or a Content-Security-Policy containing frame-ancestors, online checkers will not show the page. If neither appears, framing is allowed. In Chrome you can also open DevTools, go to Network, select the page request and read the response headers.
If it is your own site, do not weaken these headers only to please a checker: they protect your visitors from clickjacking. Test with a tool that does not need a frame.
When the frame is blocked, use a tool inside the browser
The preview above is an online checker too: it frames the page, so it has the same limit. When a site forbids embedding, the online mobile view tool says so instead of showing an empty frame, and the answer is the extension. A mobile view extension, or DevTools device mode, changes the tab itself, so the header does not matter, and logins and localhost work as in any tab. The Chrome mobile emulator guide covers DevTools, and mobile simulator extension vs DevTools helps you pick.
What a responsive checker cannot judge
A checker that only changes the width is the same as dragging the edge of your browser window. It does not tell you whether the server sends a different page to phones, how a layout behaves with long content or 200% zoom, or how Safari draws it. The full method, from the width sweep to automated screenshots, is in how to test responsive design. Treat a checker as a first look, not as sign-off.
Frequently asked questions
Why does a responsive checker show a blank page for my site?
Usually because your site sends X-Frame-Options or a CSP frame-ancestors rule that forbids framing. Check the response headers as shown above. Localhost and login pages fail for a different reason: the checker’s servers cannot reach them.
Can I test localhost with an online responsive checker?
No. An online tool cannot reach an address that exists only on your computer. Use Chrome DevTools or a mobile view extension in the tab where your local site is open.
Is a responsive checker enough to approve a site?
No. It shows layout at a few widths. Real sign-off also needs content stress tests and a check on real devices, which the responsive testing guide lays out.
Sources: MDN, X-Frame-Options (read 28 October 2026); response headers of bbc.com requested with curl on 28 October 2026.