Known limitations

ResponsiveX frames real sites inside iframes. A few things the iframe model cannot fully emulate — when a frame won't load, the diagnostic panel names the most likely cause below.

frame-busting — the site redirects out of the iframe

Some sites run JavaScript that forces top.location to escape framing. ResponsiveX sandboxes frames (no allow-top-navigation) so this can't hijack the viewer, but such a site may refuse to render inside a frame. No workaround.

csp-residual — a <meta> CSP or SameSite cookie

Header stripping (declarativeNetRequest) removes X-Frame-Options / Content-Security-Policy response headers. A CSP delivered via a <meta> tag in the HTML, or a login that relies on SameSite=Strict cookies, is outside that scope and may still block content.

network — the page is unreachable

Typically a dev server that isn't running. Check that your localhost / 127.0.0.1 server is up and serving the port in the address bar.

Other model limits

• Pixel-perfect device emulation is out of scope — the iframe uses your desktop rendering engine.
• Per-device User-Agent override changes only what client-side JavaScript sees (navigator.userAgent). The request header sent to the server is unchanged — MV3 cannot set it per sub-frame — so server-side UA detection is unaffected.
• True-viewport offsets are hand-curated; unknown devices fall back to a per-class default (editable).