A: It depends on the source. Playing Minecraft Classic or a trusted Eaglercraft mirror is generally safe, as you're not downloading files. Avoid unknown websites that ask you to download programs.
Minecraft 1.5.2 is lightweight enough to run smoothly in a browser, has been fully reverse-engineered, and sits in a sweet spot between having enough features (redstone, The End, Nether) while still being performant on low-end hardware. Later versions like 1.8 and 1.12 are now also available through Eaglercraft, but 1.5.2 remains the most mature and stable browser-based release. ---- Minecraft 1.5.2 Unblocked