HTML stands for HyperText Markup Language: it is used to create the web page itself, including text and markup commands.
Like nearly all database engines, MySQL accepts commands in Structured Query Language (SQL). SQL is the way that every user (including a PHP program) communicates with MySQL.
CSS stands for Cascading Style Sheets: styling and layout rules applied to the elements in an HTML document.
Some open source technologies are maintained by companies that accept bug reports and fix the errors like any software company. But open source software also depends on a community, so your bug report may be handled by any user who understands the code well enough. You may someday fix bugs in an open source tool yourself.