Beta sites contain the following CSS:
For people using the non-mobile interface, it appears that the B and STRONG tags can be used more or less interchangeably, and that the same is true for I and EM. However, in the mobile site CSS, only the latter of each pair is styled—only EM and STRONG work.
If you use the B or I tags they silently fail for users on the mobile site.
This wouldn't be a problem if people didn't use these tags, but they do. Why?
Because of a long-standing bug, the Markdown engine doesn't handle bold or italics correctly with Chinese or Japanese, and users often insert HTML as a workaround. But while Markdown turns stars into EM and STRONG tags, users adding HTML by hand often assume they can use the B and I tags instead. After all, they're easier to type and it looks like they work!
And since it appears to work on the full site, these users may never know there's a problem on mobile.
This problem doesn't just come up on Chinese and Japanese, either. People don't just use HTML as a workaround—they use it as a first resort. Over on ELL meta, a user recently argued that the I tag is to be preferred to EM. And it's easy to find real examples of posts on ELL using those tags. In fact, even Stack Exchange employees recommend using the B and I tags.
Take a look at these screen shots:
You can see this for yourself over on the Japanese meta, if you like. As far as I can tell, it doesn't matter which browser you use. Click back and forth between the full and mobile sites to see the difference.