Internally the masses are calculated, it’s a physical simulation, so the fuels have mass, as the fuel is burned, you lose mass, gravity pulls a little less on the shuttle as you gain altitude.
No other reports of a crash do you get an error message?
Hmmm, fair enough, but the graphics (as you fill the tanks) makes it look like it's being filled by volume.
Oh, I see you have the (Chrome) crash problem in the description, but the error is here anyway, in case it helps. Happens on second launch, right at maxQ (both launches was at around 4:1, so a bit short of orbit):
Uncaught abort() at Error at jsStackTrace (https://html-classic.itch.zone/html/9241885/AGKPlayer.js:1:40126) at stackTrace (https://html-classic.itch.zone/html/9241885/AGKPlayer.js:1:40309) at Object.abort (https://html-classic.itch.zone/html/9241885/AGKPlayer.js:3:13419) at _abort (https://html-classic.itch.zone/html/9241885/AGKPlayer.js:1:287743) at qXb (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:10:471520) at V8b (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:16:6685) at P9b (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:16:15441) at Xcb (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:5:308933) at gE (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:8:305528) at zk (https://html-classic.itch.zone/html/9241885/AGKPlayer.asm.js:4:655731) If this abort() is unexpected, build with -s ASSERTIONS=1 which can give more information.