AtCoder vs CSSBattle

AtCoder is a free web service for competitive programming contests and practice. Users solve programming problems in Algorithm Contest and Heuristic Contest formats, and can run source code in a browser rather than setting up a local programming environment. Its practice library contains about 5,000 past problems with explanations, and it supports nearly all major programming languages, including C++, Python, Rust, Java, and C#. Beginner Contests take place weekly, Regular Contests about monthly, Grand Contests five to six times a year, and heuristic contests about monthly. Real-time participation in Algorithm or Heuristic contests changes a user's AtCoder rating. AtCoder Jobs lets users apply to companies using their rating for roles including internships and experienced positions. PAST is an online algorithm skills test available in Japanese and English, with problems updated every six months. AtCoder says it has about 200,000 registered users and about 10,000 participants in real-time contests weekly. Its terms limit members to one account and prohibit account sharing, lending, or transfer.

CSSBattle is a web game for practicing HTML and CSS by rebuilding target images with the shortest possible code. Scores reflect both visual similarity and character count. You can open targets and write code without an account, but logging in is required to submit and receive a score. A new Daily Target appears at 00:00 UTC; these daily challenges have no leaderboards. Battles use separate targets and leaderboards, while private Versus battles support one-on-one challenges with friends. Submissions cannot use JavaScript, SVG, images, or external assets, and are rendered in a headless Chromium browser on Ubuntu 18.04. Learn courses add interactive levels and quizzes for beginners, intermediate learners, and experts. The free plan permits only the current day’s target. PLUS adds more than 1,100 past Daily Targets, top solutions, plugins, statistics, search and sharing, unlimited history, and a VSCode editor. CSSBattle Free costs nothing; PLUS costs $4.90 a month or $59 a year.