You can always use Unicode codepoints. For example, suppose you want to input ÿ (y with diaeresis, a rare French letter not present in Quoth's list
). First thing, you search any of a zillion resources for Unicode characters, e.g.
https://www.compart.com/en/unicode/U+00FF. There are several ways to get the code/entity.
1. From the "U+00FF", this means the code is 00FF, which is an hexadecimal number. As with decimal numbers, leading zeros can be removed, so it's "FF", just prepend &#x and append a semicolon and you're done: ÿ (case insensitive).
2. Convert the code to decimal, if it's not given in the page. In this case, since F = 15, we get 15*16+15 = 255. Now just prepend &# (note, no x here) and append a semicolon: ÿ.
3. Most pages will directly give you the HTML codes: ÿ ÿ ÿ. Just avoid the "mnemonic" one (ÿ), which is for true HTML files, and might not be supported in ebook readers, and pick any of the numeric ones.