What fonts does KA have built in?

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!