Steps to replicate the issue (include links if applicable):
On your cell phone, visit a random language Wikipedia homepage.
In our sample today, we visited the Vietnamese Wikipedia homepage.
We tried it on several browsers, the browser type is not important. We got the same results. As you can see in the attached image.
What happens?:
As you can see the page is a complete mess.
What should have happened instead?:
The page should look readable.
Software version (skip for WMF-hosted wikis like Wikipedia):
Other information (browser name/version, screenshots, etc.):
We especially tested in the state where we were not logged in, to see what the majority of users will see when first visiting the site.
We picked a random browser that we had not messed with the settings with either.
It is a big shame that many language Wikipedias will be one big mess when using a cell phone to view.
I am just making a guess that the banner size has not been tested on more than one language version.
Or perhaps the banners have only been tested on desktops.
But that ignores the fact that there's many people, a growing number, that use cell phones.
Note: you might think the website looks fine, but please give it a minute or two for the page to fully load.

