@Jdlrobson, do you think changing the width of .anonwarning would make sense.
If we add a max-width to the buttons they'll float on the left and I am not sure if that looks nice.

Change .anonwarning	Change max-width

Mm. Basically the issue happens when you make a button display block (e.g. stacked buttons) we could add a margin auto to circumvent this issue in MobileFrontend so I don't think it is a big deal.

@Jdlrobson Without media queries support it's hard to find a good compromise. My first thought was 300px as in iPhone Portrait with 10px margin on left and right. But after looking at the different screen resolutions that doesn't seem to be a good compromise. 460px for landscape seems more appropriate.
Moreover we have .mw-ui-block for buttons on screens like the one in the task description. So we can add aligning margin to those as well. While on a page like http://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=Special:UserLogin&returnto=Special%3AWatchlist&warning=mobile-frontend-watchlist-purpose the buttons stay aligned left.

Change 272887 had a related patch set uploaded (by VolkerE):
mediawiki.ui: Buttons should receive max-width

https://gerrit.wikimedia.org/r/272887

Change 272887 merged by jenkins-bot:
mediawiki.ui: Buttons should receive max-width

https://gerrit.wikimedia.org/r/272887