The standard symbol table of ASCIIMathML.js does not contain many symbols. It can be extended by adding additional symbols on any webpage that requires them. This is done by adding a few lines of JavaScript code.
For example, suppose we want to add symbols for "not less or equal" and "not greater or equal".
We first have to find the four-digit hexadecimal Unicode value for these symbols by looking them up at, say, http://www.w3.org/TR/MathML2/chapter6.html#chars.entity.tables
Next we have to decide what input strings we want to associate with these symbols, say "!<=" and "!>=".
Finally we add the following lines to the head or body of our HTML file:
<script type="text/javascript">
define("!<=","\u2270")
define("!>=","\u2271")
</script>
Here we test the modified symbol table: \`a !<= b !>= c\` produces `a !<= b !>= c`
To add a symbol to the LaTeX commands, use the same syntax (but with commands that start with a \):
<script type="text/javascript">
define("\\nle","\u2270")
define("\\nge","\u2271")
</script>
Now \$a \nle b \nge c\$ produces $a \nle b \nge c$.
If you know the numeric entity reference of the symbol you want to use on an ASCIIMathML webpage, you can also refer to the symbol directly by using that reference. E.g \`≰\` produces `≰`. If a symbol is only used occasionally, this is certainly the simplest way to include it.