https://archive.org/details/software...msdos_games/v2
2400 MSDOS games on this site are offered legally. They use an in-browser version of Dos-Box. You must install nothing, just click to start.
I am aware of the legal status of abandonware and Neal'S stand on it, and that abandonware does not mean automatically that it is legal to download something, it only means that the former holder of rights does not care to enforce his rights anymore.
But this company offers to play in the browser, and in this German article in a major newspaper gives me the strong impression that it all is legal:
http://www.welt.de/wirtschaft/webwel...erfuegbar.html
If it were not legal, that would be as surprising as for example the Times advertising in a major article some pirated software. Unlikely.
It looks all okay to me.