A "little indie developer" hasn't been able to write a web browser in several decades. An implementation of 2005-era CSS and JS would be a complex task surely requiring a fairly sizable team to implement.
The fact is, all three major browser implementations are open-source and that has allowed any company to come in and release or embed their own browser with minimal effort. Chrome/Blink is dominant but that is not due to technical barriers that make it difficult for other companies to ship their own browsers. In fact it is now easier than ever to do so.