Any character defined in the ISO or ANSI character set standard can be posted to html using the decimal character number in the form:
where & is the html "escape" to designate that a code follows
the number symbol # designates a numerical form for the code
nnn is the decimal number assigned to the character
a semicolon ; ends the code.
An important correction: One must use the Unicode values. ISO(-8859-1) and ANSI character values largely coincide with Unicode values, but there are also many differences.
Earlier html standards do not specifically support the use of decimal numbers larger than "three digits," although they often work okay up to four.
Any character defined by the Unicode standard can be posted to html using the hexadecimal number assigned to that character by the Unicode Standard, in the form:
where & is again the html "escape" to designate that a code follows
# designates that the code is a number
X says that the number is in hexadecimal format
hhhh is the hexadecimal number
It is entirely possible, and perfectly correct, to type:
Mary  had  a
And if I actually do that, you will see:
Mary had a little lamb
Just in case somebody doesn't see it, I posted (in hex char code) "Mary had a little lamb."
Obviously it's not necessary to "code" characters you can just type on your keyboard in most cases.
Arful Codger will explain when it doesn't work to just type them (?).