The question behind are browser games safe is usually will this give me a virus. For a game running inside a browser tab, the answer is almost always no, and the reason is architectural rather than a matter of trust.
What the sandbox prevents
Code running in a tab cannot read your files, install software, access other tabs, or reach anything else on your machine. That is the browser security model, and it is enforced regardless of whether the site is trustworthy.
- No file system access without an explicit file picker that you operate.
- No installing anything. A page cannot put software on your computer.
- No reading other sites. Same-origin policy stops a game reading your email tab.
- No access to your camera, microphone or location without a permission prompt.
What is actually worth worrying about
The real risks are not technical exploits. They are ordinary web problems:
- Advertising inside the game frame, including unskippable video before the game starts.
- Third-party tracking. Ad networks embedded in a game see that you loaded it.
- Downloads. A page offering an exe or an installer is not a browser game any more, and that is the point at which the sandbox stops protecting you.
- Sign-in prompts. A game has no reason to ask for an account on another service.
The sandbox protects you from the code. It does not protect you from being persuaded.
What we measure, and what we cannot
We load every game in a real browser and record which ad networks it contacts, then remove the ones running advertising inside the frame. Two thirds of an unscreened catalogue turns out to be doing it.
What no screening can promise is permanence. These are other people websites. A publisher who was clean last month can add an ad stack tomorrow, which is why screening has to be repeated rather than done once.
Practical advice
Do not download anything a game offers you. Do not enter an account password into a game frame. If a game opens new tabs on click, close it. Everything else is noise.