I noticed that the site is responsive, having a viewport meta tag would make it more convenient for mobile browsing.
Posted under General
Strange that it was never implemented before.
This is now added in commit 5de709f
I can't say for sure when it'll be pushed live though.
http://danbooru.donmai.us/forum_topics/9127?page=99#forum_post_109258
Looks like it was reverted not long after being added because it forcibly enables the mobile layout but the mobile layout sucks.