Terminal
Pick a system and type help. Pipes, redirection, globs and $VAR all work, and the filesystem survives a reload. Three of the four are written here and answer instantly. The fourth is a real Linux kernel booted in this window, and it says what it will download first.
Settings and the filesystem are kept in this browser only.
Tab completes, Up recalls, Ctrl+C interrupts, Ctrl+L clears. Long output opens a pager: space, b, / to search, q to leave.
On the real machine every key goes to the guest while the window has the keyboard, and Esc gives it back. Inside the guest, Ctrl+[ is Esc. The machine keeps running while you look at the other three, and it is saved when you leave the page.
Whole machines with screens of their own, from a boot sector to a desktop, are on the machines.