Online Non-Downloadable Multiplayer Games: Browser/Instant Play Options

These run in-browser (HTML5/WebGL/WebAssembly) — no install, cross-platform:

Strategy / Board / Card

• Chess.com / Lichess.org – Real-time & correspondence.
• Tabletopia / Board Game Arena – 100s of licensed board games (Catan, Wingspan, 7 Wonders).
• Playdek – Ascension, Pandemic, Twilight Struggle.
• CardzMania / Trickster Cards – Hearts, Spades, Euchre, Oh Hell.

Action / Arcade / .io

• Agar.io / Slither.io / Diep.io / Paper.io – Classics.
• Krunker.io – Fast FPS (runs on potato).
• Shell Shockers – Egg FPS.
• EvoWars.io / Hole.io – Growth arena.

Social / Deduction / Party

• Skribbl.io / Gartic Phone – Drawing/guessing.
• Among Us (Web) – amongus.io (unofficial) or Steam Deck / GeForce Now for real version.
• Town of Salem / Throne of Lies – Browser werewolf.

MMO / Persistent

• RuneScape (Old School / RS3) – Full MMO in browser (Jagex launcher now preferred).
• AdventureQuest 3D / DragonFable – Flash/Unity legacy.
• BitHeroes / Realm of the Mad God (RotMG) – Bullet-hell MMO.

Platforms

• CrazyGames / Poki / Y8 / Kongregate (HTML5) / Newgrounds / itch.io (Web) – Curated libraries.

Mobile: "Instant Games" in Facebook Messenger / Discord Activities (Poker Night, Chess, Wordplay).