Tiny proof of work library for browser. It does use SubtleCrypto and async/await so it requires a secure context and a modern browser.
<scripttype="module">importtinyPowfrom"https://unpkg.com/tiny-pow";// use it as a module or just copy/paste the codeawaittinyPow("my data",4)// will return the salt that has 4 zeros at the beginning of sha256("my data" + salt)</script>