Hash any text with MD5, SHA-1, SHA-256, SHA-384 and SHA-512 at once, in hex or base64, computed in your browser.
Because the usual reason to open this page is a checksum whose algorithm nobody wrote down. Seeing all five means you can match it by length and value rather than by guessing.
No. A password needs a slow hash with a salt, such as bcrypt, scrypt or argon2. SHA is built to be fast, which is exactly what makes it the wrong tool there.
SubtleCrypto only implements the algorithms it considers safe, so MD5 is implemented here from RFC 1321 and checked against the standard test vectors. It is for reading old checksums, nothing else.
Usually a trailing newline, or line endings: a file saved on Windows carries \r\n and hashes differently from the same text with \n. Encoding matters too, since this hashes UTF-8 bytes. Hash the file rather than its text if the two must agree.