Back in 2008, Dr. Drang wrote about how Monospace Characters in iOS were not actually monospaced. The situation has improved since then.
Today, drawing the same characters in a <pre> or <code> block works just fine.
...with an important exception. Using certain unicode "line drawing" characters in a <pre> or <code> block still, maddeningly, does not appear to be monospaced.
Here's an example of line drawing, which should look good on a desktop browser:
┌───────┐ │·······│ │·······│ │·······│ │·······│ └───────┘
When I previewed my line drawings on iOS, I couldn't get them to look right.
After searching the web I did find a little help.
Courier New appears to do a much better job of handling extended characters, and by explicitly requesting "Courier New" in your stylesheet, your text will render correctly. Here's a demonstration: