I guess these are the built in ones, if you can call them that: sans-serif, serif, monospace, fantasy, cursive.

source: exec-pjs.js

println(PFont);
/*
function PFont(e,t){
    if(e===qr){e=""}
    this.name=e;
    if(t===qr){t=0}
    this.size=t;
    this.glyph=false;
    this.ascent=0;
    this.descent=0;
    this.leading=1.2*t;
    var n=e.indexOf(" Italic Bold");
    if(n!==-1){e=e.substring(0,n)}
    this.style="normal";
    var r=e.indexOf(" Italic");
    if(r!==-1){e=e.substring(0,r);this.style="italic"}
    this.weight="normal";
    var i=e.indexOf(" Bold");
    if(i!==-1){e=e.substring(0,i);this.weight="bold"}
    this.family="sans-serif";
    if(e!==qr){
       switch(e){
            case"sans-serif":case"serif":case"monospace":case"fantasy":case"cursive":
            this.family=e;
            break;
       default:
            this.family='"'+e+'", sans-serif';
            break
       }
    }
    this.context2d=null;
    h(this);
    this.css=this.getCSSDefinition();
    this.context2d.font=this.css
}
PFont.prototype.getCSSDefinition=function(e,t){
    if(e===qr){e=this.size+"px"}
    if(t===qr){t=this.leading+"px"}
    var n=[this.style,"normal",this.weight,e+"/"+t,this.family];
    return n.join(" ")
};
*/

Not sure what that is.
What I mean is:
What fonts does KA use on the website that are built into the website that will look the same one every computer

I believe those would be sans-serif, serif, monospace, fantasy, and cursive. 

Fantasy, cursive don't work on my computer

Hmm, that’s odd. I was pretty sure that those were the fonts built in, but I guess it might just depend on what device you have. Maybe try placing a help request on one of your programs, or asking a question on a relevant video/article relating to text (make sure you say that fantasy and cursive don’t work on your computer).

List of KA built-in fonts:

• Lato
• Source Serif Pro
• FontAwesome
• FreeMono
• PjsEmptyFont

Source: I used my browser font tool to see what font came from where.

What fonts my browser used:
Name (alternative name) – from where

Menlo Regular - System
Monaco - System
Courier (monospace) - System
Apple Chancery (cursive) - System
Papyrus (fantasy) - System
Times Roman (serif) - System
Lucida Grande - System
LatoLatin Regular (Lato) (san-serif) - Remote
LatoLatin Bold (Lato bold) (san-serif Bold) - Remote
(PjsEmptyFont) - Remote

Available remote fonts through CSS @font-face):
"file name"
font-family -
src

"ef51404934aee86faaf2654970b8b466.0a3a86628abd22d7c375.css"
Lato -
url(/fonts/LatoLatin-Regular.woff2) format('woff2')
url(/fonts/LatoLatin-Italic.woff2) format('woff2')
url(/fonts/LatoLatin-Bold.woff2) format('woff2')
url(/fonts/LatoLatin-Black.woff2) format('woff2')
Source Serif Pro -
url(/fonts/SourceSerifPro-Regular-Extended.woff2) format('woff2')
url(/fonts/SourceSerifPro-Regular.woff2) format('woff2')
url(/fonts/SourceSerifPro-Semibold-Extended.woff2) format('woff2')
url(/fonts/SourceSerifPro-Semibold.woff2) format('woff2')
FontAwesome -
url('/fonts/fontawesome-webfont.woff2?v=3.6.7') format('woff2')
url('/fonts/fontawesome-webfont.woff?v=3.6.7') format('woff')

"dc4f1cea1e731c1f7f8e0ff07b58da2b.8db5d598f11a07f48ab2.css"
FreeMono -
url('/fonts/FreeMono.woff?20120503') format('woff')
url('/fonts/FreeMono.ttf?20120503') format('truetype')
url('/fonts/FreeMonoBold.woff?20120503') format('woff')
url('/fonts/FreeMonoBold.ttf?20120503') format('truetype')
url('/fonts/FreeMonoOblique.woff?20120503') format('woff')
url('/fonts/FreeMonoOblique.ttf?20120503') format('truetype')
url('/fonts/FreeMonoBoldOblique.woff?20120503') format('woff')
url('/fonts/FreeMonoBoldOblique.ttf?20120503') format('truetype')

"intro.d0fe2d3794d2a5d4f67f.css"
FontAwesome -
url('/fonts/fontawesome-webfont.woff2?v=3.6.7') format('woff2')
url('/fonts/fontawesome-webfont.woff?v=3.6.7') format('woff')

<inline>
PjsEmptyFont -
url('data:application/x-font-ttf;base64,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') format('truetype')

The built-in fonts on Khan Academy are:

• Mono/Monospace
• Cursive
• Fantasy
• Serif
• Sans-Serif

The reason Cursive, and Fantasy were not working is that they are case sensitive so make sure the first letters of them are lowercase!

Garamond is included as well for PJS programs:

textFont(createFont("Garamond"));
fill(50, 50, 50);
textSize(90);
text("Text", 120, 210);

Garamond = sans-serif or at least I think 
`

textFont(createFont("Garamond"));
fill(50, 50, 50);
textSize(90);
text("Text", 120, 210);
textFont(createFont("sans-serif"));
fill(50, 50, 50);
textSize(90)
text("Text", 118, 308);

I've made a list of all the fonts I know of and how to use them. Hope It's helpful. :)

https://www.khanacademy.org/computer-programming/the-library-of-fonts/4676057911705600