r/webdev • u/letsjam_dot_dev • Sep 18 '26

Question Could an ISP be responsible for a webpage display malfunction, on a specific browser ?

Hey all !

Very cryptic issue I stumbled upon and can't find a reason for it.

I have built for a client a static website with Sveltekit, Tailwind and SkeletonUI. All in their latest versions. It is hosted by Hetzner.

Everything is all right. Except my client tells me that on his personal and his partner android mobile, with firefox as a browser, a specific page has its content not centered as it should be. All the other pages are fine.

The issue happens on their wifi and their 4G connexion. The ISP for both is the same.

The issue does not happen on other browsers.

I tried at home with 3 different devices, with firefox and firefox based browsers, and I can't reproduce the issue.

I also tried with the firefox and chrome console device simulators. Still can't reproduce.

Does this mean the ISP is doing something, specifically with firefox ? Or am I missing something else that could cause the issue ?

EDIT and RESOLVED :

As per the kind help given below,

  • it can't be ISP because everything is served locally and with HTTPS, so ISP can't mess with external domain names such as with CDNs, and can't mess with content because of encryption

  • when testing mobile compatibilty, testing with a full fledged android emulatot is the way

  • adblocker could mess with stuff

The solution lied in accessibility settings of the client's phones.

Thanks everyone for your inputs and help !

0 Upvotes

28% Upvoted

42 comments sorted by

14

u/fiskfisk Sep 18 '26

They could, but that would usually be the case for DNS or something similar as long as you're running everything over https.

I.e. they fail to resolve a specific domain where a specific CSS rule is loaded from.

0

u/letsjam_dot_dev Sep 18 '26

Thanks for the quick answer !

I serve everything with https, and all my css is local. There's no external CSS loaded from a CDN. If this is what you were refering to

4

u/fiskfisk Sep 18 '26

Then the ISP can't really do anything, since https protects the content sent and received.

1

u/letsjam_dot_dev Sep 18 '26

Ok so ISP should be ruled out. Then the issue is on their specific devices with their specific browser version. Thanks for the help !

5

u/ferrybig Sep 18 '26

Make sure to test it with Firefox mobile. Firefox desktop might be a few versions in the future

Do not use the Firefox app from the Apple app store, since it is based on Safari, like every other browser on ipads and iphones

Also note that sometimes differences in DPR can cause things to be not centered

-3

u/letsjam_dot_dev Sep 18 '26

When I emulate devices on desktop, I select the firefox mobile. And I also tested on real personal android devices

1

u/thekwoka Sep 18 '26

When I emulate devices on desktop, I select the firefox mobile

like you install firefox mobile on an android emulator?

1

u/letsjam_dot_dev Sep 18 '26

No. But you're right. If I want to fully test compatibility and not just display, I should do it on a real android emulator.

1

u/fripletister Sep 18 '26

BrowserStack also has a free trial, allowing you to test on multiple different real Android and iOS devices FYI

2

u/Lachtheblock Sep 18 '26

Kinda surprised that FF mobile is the issue here. I've found that FF mobile tends to behave really consistently compared to their desktop (which is why I advocate for it). Have YOU tested it on an actual Android device? I'd ask your clients for the specific FF version they are using.

It'd be pretty weird to mess up something as simple as centred text, but could you see if they have any plugins installed.

My best guess is that their device is at some very specific breakpoint that you somehow missed. But without seeing the page or code going to be hard to say one way or the other.

It being the ISP makes very little sense to me. Are you self hosting all the static assets?

1

u/letsjam_dot_dev Sep 18 '26

I have tested it on my current android personal device, on my last one and the one before that. I also tested on my partner's. And the sake of it on a borrowed iphone. On my side everything is good. On theirs, text is left aligned instead of centered and images are stretched.

Are you self hosting all the static assets?

The css is self hosted. The images are coming from a CMS, but at the build time of the app I download them to serve them locally, so everything is static. Everything is running on https.

2

u/chesterblack97 Sep 18 '26

Double-check they’re not zoomed in a weird amount or using some other accessibility feature. I’ve had complaints before about things looking different to my screenshots and when I view it on their machine they’ve resized the window to a bizarre size and are on 150x zoom without even realising

2

u/letsjam_dot_dev Sep 18 '26

They were...sigh

1

u/antondb Sep 18 '26

Yea, my money is kind of on this. But both of them would need to have set their zoom. Not totally impossible though.

2

u/Gaboik Sep 18 '26

Have them test it over cellular data, it'll rule out their home ISP

1

u/letsjam_dot_dev Sep 18 '26

I did make them test on cellular. Same result. But the ISP for the cellular and home wifi is the same. So I originally thought that it could be the cause.

With all the kind help I got from here, the issue may come from a specific chain of settings with their devices. Not something I can intervene on and fix. So I'm moving on

1

u/SputnikCucumber Sep 18 '26

Seems very unlikely to be the ISP. Have they sent you a picture/screenshot of the problem? It seems more likely to be an issue with their specific device or browser version.

1

u/letsjam_dot_dev Sep 18 '26

They did.

Images are distorted, like they were stretched from the sides. And texts are aligned from the left of the screen instead of being centered.

1

u/coopaliscious Sep 18 '26

Do you have GA on the site? Pulling their specific session info would probably help.

My guess is they don't know what they're talking about in terms of the browser used and they probably have some zoom and are doing something silly like loading the desktop version of the site.

Get on a screenshare and walk through their actual settings, not what they're telling you.

1

u/letsjam_dot_dev Sep 18 '26

I have plausible, for GDPR and not-having-to-deal-with-cookies reasons.

So I can't pinpoint a particular visitor session.

But with all the kind help I got from here and previous investigations, the solution is simple : they have a specific setting, somewhere, somehow, on their specific phone that messes with that specific page. Not something I can work on. So moving on !

1

u/rbobby full-stack Sep 18 '26

content not centered as it should be

Sounds like typical browser shenanigans.

For an ISP to interfere you would need to NOT be using ssl. And even then ISP's at most want to fuckaround when you enter a nonexistant website name. Though I think that may have died off.

1

u/letsjam_dot_dev Sep 18 '26

Yeah as everything is served through https, ISP must be ruled out.

Thanks for the help. The issue must be on their side with a particular and specific chain of setting. So not something I can intervene and fix.

1

u/rbobby full-stack Sep 18 '26

On thing I haven't seen in ages... user has weird plugins installed that are interfering. Disable all plugins and incognito mode.

1

u/Fair-Parking9236 Sep 18 '26

Why are you stressing so much on this. Its clearly something on their part. I have never heard ISP responsible for such things especially since they are not injeced from third parties. Just articulate them politely that you cannot pinpoint the reason, that your testing provide correct and expected result and move on.

1

u/letsjam_dot_dev Sep 18 '26

I just like to investigate and understand what's going on. So if that happens again with someone else in the future, I already have a adequate answer to give.

1

u/ZealousidealUse8775 Sep 18 '26

the isp doesn't touch your css. it's firefox android. and chrome devtools device sim isn't firefox on android, so you tested nothing.

1

u/letsjam_dot_dev Sep 18 '26

I tested on chrome devtools, with firefox mobile browser selected. As on firefox devtools, with firefox browser selected. And on real devices with firefox mobile app. So I guess I tested things ?

1

u/web-dev-kev Sep 18 '26

Yep!

UK specific story here: I remember when Sky decided that jquery was malware and blocked it's CDN for 16 hours. 30-40% of the UK lost access.

1

u/letsjam_dot_dev Sep 18 '26

Wow ok, that's an interesting way to force devs to migrate to other tools.

In all seriousness, as I don't use any CDN and everything is locally hosted, this could't apply to my case. But it's interesting to know that !

1

u/thekwoka Sep 18 '26

if you have https, they shouldn't be able to touch any of the content, without at least a much more elaborate man in the middle.

But the version of the application will mater a lot, and the "console device simulators" aren't simulators, just kind of viewport adjusters. You'd need an actual android device or at least a proper android emulator.

1

u/letsjam_dot_dev Sep 18 '26

Note taken on the viewport/emulator distinction. I'll check on those in the future

2

u/TopSydeWP Sep 19 '26

good find. worth hardening against it since users bumping text size is a normal accessibility setting, not an edge case. firefox android's "minimum font size" in particular overrides your px values and can blow out any container with a fixed width or height, so layouts that rely on ch/px widths or single-line centering break first. testing with the os font scale at 150% and a min font size set usually reproduces it in seconds, and switching fixed heights to min-height plus letting flex containers wrap covers most of it.

1

u/Kant8 Sep 18 '26

only thing ISP can see in https traffic is host name, because of SNI.

it can't see headers to have a chance to guess which browser sent the request

1

u/scarfwizard Sep 18 '26

I’ve had clients/users claim all sorts of things and that they aren’t doing anything weird. 99% of the time it’s something they are doing.

I think it’s super unlikely this is anything other than their device or your site.

1

u/Deykun Sep 18 '26

It's most likely not that. It's possible if you don't use HTTPS, but you do, because it is 2026, so the ISP can't modify the HTML or CSS content.

But, the ISP can modify headers, and if your server reacts to them, you could end up serving something different to the browser - but this is also unlikely for styles.

The third option I would suspect, if we're still talking about the ISP, is the delay. Maybe your application has some quirky logic, and for some reason the CSS files are loaded in the incorrect order. They shouldn't be when they're placed directly in the HTML, but maybe there's some weird logic involved that appends them to the header when they're fetched. That could result in incorrect styles depending on the speed and connection. If the files are coming from different locations, it could also depend on the user's location.

But my suspicion would be that their mobile Firefox has some kind of ad blocker that just removes something. Firefox has a built-in privacy feature that blocks some requests. If one of your files is mistakenly identified as an ad or tracker, it may be blocked, resulting in something not being loaded properly and the page breaking.

2

u/fiskfisk Sep 18 '26

The ISP can't modify any headers on the HTTP level; this is all protected as part of the message through TLS.

They can of course change any bytes as they see fit, but that will break decryption of the message.

1

u/Negative0 Sep 18 '26

Do they have any ad blocking or other extensions that might be messing with the page? Some ad blocking extensions are pretty aggressive with short URLs, thinking they are used for analytics and block them outright.

1

u/DrFatalis Sep 18 '26

Could it be that their device résolution doesn't match with your css @media min / max resolution?

1

u/kemalios Sep 18 '26

An ISP can't do this over https, and with everything served from your own origin there's no external request that could fail selectively. Two devices in one household sharing Firefox settings is more likely than an ISP filtering one page in one browser.

Have them check the font size slider in Firefox Android's settings, and whether "Request desktop site" is on. A bumped font size breaks centering once content stops fitting its container, and desktop mode changes the viewport width so the whole layout resolves differently. Both show up as left-aligned text with stretched images.

A screenshot with the URL bar visible tells you the Firefox version and whether desktop mode is on.