It certainly is now (HTML5 retroactively standardized a lot of quirks), but it wasn't always well-defined: https://scrappy-do.blogspot.com/2004/08/little-rant-a

It certainly is now (HTML5 retroactively standardized a lot of quirks), but it wasn't always well-defined: https://scrappy-do.blogspot.com/2004/08/little-rant-about-microsoft-internet.html https://scrappy-do.blogspot.com/2004/08/little-rant-about-mi...