Formatting
Paste code or open a file and it is laid out as you type: one statement or declaration to a line, with each level indented by 2 or 4 spaces or a tab. Auto works out the language from how the code starts: a < means HTML, a word like SELECT or CREATE TABLE means SQL, a selector followed by { and a property means CSS, and anything else is read as JavaScript. If it guesses wrong, pick the language yourself. HTML, CSS and JavaScript are laid out by js-beautify, and the CSS and scripts inside an HTML page are laid out too.
SQL dialects
SQL is laid out by sql-formatter, which puts each clause (SELECT, FROM, WHERE, GROUP BY) on its own line with its list indented under it. Pick your database under Dialect so its own syntax is understood, such as backtick names in MySQL, square brackets in SQL Server or $$ strings in PostgreSQL. Keywords sets whether words like select are written in capitals, in lower case or as you typed them; table and column names are never changed.
Minifying
Minify takes out comments, line breaks and spaces that do not change what the code does, and says how many bytes that saved. JavaScript goes through Terser, which also shortens local variable names and simplifies expressions; names other code can see, such as top-level functions, are kept. CSS, HTML and SQL are squeezed by rules written for this page that only remove whitespace and comments: HTML keeps one space between words and inline tags, so text does not run together, and leaves <pre> and <textarea> as they are. Comments starting /*! are kept, since they usually hold a licence.
Questions
Is my code sent anywhere?
No. The formatters are loaded into this page the first time you use them and run on your device. Nothing you paste is uploaded or saved, so private code and connection strings stay with you.
Why did my JavaScript not minify?
Terser has to read the code before it can shrink it, so a syntax error stops it; the line and column of the problem are shown. Formatting still works on broken code, since js-beautify lays out whatever it is given. TypeScript and JSX are not plain JavaScript, so they can be formatted but not minified.
Does it check my code for mistakes?
Only as far as it needs to. SQL that sql-formatter cannot read is reported, and JavaScript with a syntax error will not minify, but nothing here runs your code or checks that it does what you meant.
Will minified HTML look the same?
It should. Spaces are kept between words and inline elements and dropped only beside block elements such as <div> and <p>, where a browser does not draw them. Scripts inside the page are trimmed but not minified; minify them as JavaScript first if you need them smaller.