I built a static website for a client using SvelteKit, Tailwind, and Skeleton UI, and it is hosted on Hetzner. The site works correctly almost everywhere, but one particular page has a layout problem on my client's Android phones when viewed in Firefox. The text appears left-aligned instead of centered, and some images look stretched. All other pages work normally.
The issue occurs both on their home Wi-Fi and on 4G, which use the same ISP, but it does not appear in other browsers. I have tested several Android devices, Firefox-based browsers, desktop browser device simulators, and an iPhone without reproducing it. The site uses HTTPS, the CSS is hosted locally, and the images are downloaded from the CMS during the build so the deployed site is fully static.
Could the ISP be altering or selectively serving something to Firefox, or is this more likely to be caused by a browser version, device setting, extension, breakpoint, or something in the page's CSS?
4 Answers
Start by treating this as a device-specific Firefox issue. Ask for the exact Firefox version, Android version, screen size, display scaling, and any installed extensions or privacy settings. Firefox’s tracking protection or an ad blocker could theoretically block a resource, although a breakpoint or CSS rule that behaves differently at that device’s viewport is more likely. Device pixel ratio and browser zoom can also expose a missed responsive breakpoint.
An ISP-based explanation is very unlikely here. With HTTPS, the ISP cannot normally inspect or modify the HTML or CSS contents, and it would not be able to selectively rewrite a page based on the browser in the way you describe. At most, DNS or connectivity problems could prevent a resource from loading, but you said the assets are local and served over HTTPS.
The CSS and images are served locally from the static build, and everything uses HTTPS, so there do not appear to be any external stylesheets or CDN assets that could be blocked.
Try reproducing the problem on the client’s exact phones while connected to their network, then compare the loaded page source and computed styles with a working device. Browser extensions, cached assets, accessibility settings, forced font or page scaling, and Firefox privacy protections are all worth checking. If only their devices show the issue, collecting the exact model, Firefox version, viewport dimensions, and a remote inspection of the affected page should narrow it down much faster than investigating the ISP.
The screenshots suggest that some layout rule is not being applied, rather than an ISP changing the page. Check whether the affected page receives the expected stylesheet, whether any CSS request fails, and whether a media query or container rule changes at that phone’s exact viewport width. Also inspect image sizing rules such as width, height, object-fit, and max-width. A missing class, malformed CSS, or a selector that only fails at one breakpoint could explain both the alignment and stretching.
Make sure the comparison is done in actual Firefox for Android, not just a desktop device simulator. Mobile Firefox can differ from desktop Firefox, and the exact browser version matters.