Here you can play game Half-Life: Deathmatch Windows in browser online.
Attention: servers have already been created. All you need is to go to Multiplayer->Internet Games->Refresh and select one of the currently created games. Also, when starting the game, you can specify a different url server socket, if you know which one can connect to.
It will also be useful to change the screen resolution in the game to fit your monitor. To do this, go to the Configuration-> Video-> Video modes section and select a screen resolution convenient for you. Important: after changing the resolution, press the blue button "+" or "-". Then the screen will automatically be adjusted to fit the game window. And then click "Fullscreen".