C# and Java: The Plan, and the One Dependency It Introduces
Both have a free, workable path to running in the browser. Java's comes with a real trade-off worth stating plainly.
C# is planned on Blazor WebAssembly -- real .NET running in the browser, Microsoft-maintained, free, and deployable as static files. Its cost is first-load weight, heavier than everything else on this site combined, so it will likely load only on the pages that actually need it rather than sitewide.
Java is planned on CheerpJ, a mature, actively developed JVM-in-the-browser project. Its free community licence covers a one-person company -- which is what Coders Rock is and is expected to stay -- at no cost, but on one real condition: the runtime has to load from CheerpJ's own hosted CDN rather than being vendored the way the Python interpreter is. That makes Java challenges the one place on this site where a third party would sit in the request path to load the runtime, even though the code you write still runs, and never leaves, your own browser once it's loaded. Worth saying plainly rather than quietly softening the site's own claim about where your code goes.
Everything above runs in this page, on your own machine. There is more of it on the practice pages.