https://archive.org/details/internetarcadePlay hundreds old arcade games for free in your browser! (I don't work for them. Saw it on reddit).
Generic instructions:
" Pressing the 5 key will insert a coin, and pressing 1 or 2 will represent Player 1 Start and Player 2 Start. Generally, arrow keys are used for joystick directions or to act like a paddle, and the CTRL, ALT/OPTION, and SPACE keys are for various buttons. If you wish to see what the current key mappings are for a system, press the TAB key and go to the key mappings menu."
