It has already been discussed on the general site meta at Whitelist the <span> tag with the lang= attribute, in order to support Han characters, but I'd like to draw attention to it again here at JSE, given that we are the site most affected by this.
Lately there have been a few questions asking about variant character forms. This is only natural, given the divergence of Chinese and Japanese simplification schemes, all thrown together with other regions not simplifying their Chinese characters (hk, tw). Even so, it makes it incredibly difficult to answer these questions, because JSE only has proper support for Shinjitai in unicode. This means that when I try to type 直, no matter what I do, it comes out with an L shape, even if I want to show the form with a flat bottom line.
More ridiculously, these characters turn up in their Chinese form in the title (but ONLY in edit mode) and URL bars. While I'm aware that the latter can't be helped, character forms in titles is definitely within our capacity as a website to change. In fact, to add to this, if I make a heading or use sub/superscript, 直 comes out...
The important thing for our site is
(1) The ability to properly display Japanese characters
(2) To show variant character forms
Currently, characters often come out in their Chinese form, as shown above, meaning that the site isn't able to display Japanese as a Japanese language site (this means we haven't achieved (1)), and we also can't show variant forms used in unicode so that we can answer people's questions about why characters look different in URLs and elsewhere on the internet (meaning we haven't achieved (2)).
The simplest way to approach this is to allow this using the < span > tag, so that we can write
<span lang="zh">隆</span> <span lang="ja">隆</span>
and get display two different characters (that share a unicode codepoint), one for each language.
This is an essential feature for JSE. In its current state, it doesn't properly show Japanese characters in titles and scripts, while equally it can't show variant characters to answer questions from people wondering about encoding and typing, calligraphy, handwriting, and printing of these variants.
Questions that have asked about variant forms (I think these shed light on the necessity of this feature):
How to find alternative writings of a kanji in electronic form?, not specifically about character forms, but was strongly affected as a question because of JSE's handling of variants