The default APL font for the site is SImPL medium, SImPL, or APL385 Unicode.
If the APL characters in the articles don't display correctly, you can obtain a copy of the SImPL medium font by downloading the current NARS2000 binary, or the SImPL font from here, or the APL385 Unicode font from here.
If they still don't display correctly, download FireFox.
Even if you have an APL font installed, that may not be the end of the story. Each font designer decides what glyphs to display and for which characters. Also, some implementations of supposedly Unicode fonts don't always display the same glyphs in the same places or in the same way. The SimPL Medium font, which comes with NARS2000, will display some characters with different glyphs than are displayed by the font APL385 Unicode. For example, these are the underlined letters A to Z, which are valid identifiers in APL: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z. Now, to display these characters in APL385 Unicode, the characters use code positions 9398 through 9423, or 0x24B6 through 0x24CF, which show up as Ⓐ Ⓑ Ⓒ Ⓓ Ⓔ Ⓕ Ⓖ Ⓗ Ⓘ Ⓙ Ⓚ Ⓛ Ⓜ Ⓝ Ⓞ Ⓟ Ⓠ Ⓡ Ⓢ Ⓣ Ⓤ Ⓥ Ⓦ Ⓧ Ⓨ Ⓩ, which, while not the same, is very close. When copying underlined APL characters to the clipboard, you may convert them to lowercase by first setting the user preference "Underbar alphabet converted to lowercase".
The standard Arial font doesn't even have a built-in set of underlined characters, nor does SimPL. (Yet note some glyphs, as shown below, that are used by NARS2000, are not present in APL385.)
Also, the settings for Unicode for APL symbols, which are listed as being 0x2300 - 0x23F3, do not necessarily appear - or appear the same - in all APL fonts.
Extra APL Glyphs
While any Unicode APL font will display all of the standard APL glyphs, NARS2000 uses (or has plans to use) some extra glyphs not found in all Unicode APL fonts such as
|Section||§||167 or U+00A7||Alt-S||Multiset Symmetric Difference|
|Pi||π||960 or U+03C0||Alt-p||Prime Decomposition and Number-theoretic|
|Root||√||8730 or U+221A||Alt-R||Square Root and Nth Root|
|Infinity||∞||8734 or U+221E||Alt-f||Infinity|
|Mismatch||≢||8802 or U+2262||Alt-@||Mismatch|
|Left-shoe-underbar||⊆||8838 or U+2286||Alt-Z||Subset of|
|Right-shoe-underbar||⊇||8839 or U+2287||Alt-X||Superset of|
|Circle-middle-dot||⊙||8857 or U+2299||Alt-?||Null operator|
|Quad-colon||⍠||9056 or U+2360||Alt-_||Variant operator|
|Dieresis-downtack||⍡||9057 or U+2361||Alt-N||Convolution operator|
|Dieresis-del||⍢||9058 or U+2362||Alt-G||Dual operator†|
|Dieresis-star||⍣||9059 or U+2363||Alt-P||Power operator†|
|Dieresis-jot||⍤||9060 or U+2364||Alt-J||Rank operator|
|Dieresis-circle||⍥||9061 or U+2365||Alt-O||Composition operator|
|Down-shoe-stile||⍦||9062 or U+2366||Alt-m||Multiset operator|
|Dieresis-tilde||⍨||9064 or U+2368||Alt-T||Commute operator|
|Del-tilde||⍫||9067 or U+236B||Alt-k||Commutator operator†|
|Zilde||⍬||9068 or U+236C||Alt-}||Empty simple numeric vector|
|Stile-tilde||⍭||9069 or U+236D||Alt-M||Partition operator†|
† = Not currently implemented.
All of the above glyphs may be found in SImPL medium, all but the Infinity symbol in SImPL, and all but the Null operator in APL385 Unicode.