Here are the commandments for web design. I may add to this list as I think of new stuff.
1. Thou shalt not have autoplay videos or sounds.
Seriously. It's annoying. Any web designer who violates this commandment shall be sentenced to have the video or audio play in their house simultaneous with each time it autoplays for a visitor to their website.
Here, I'm talking about "var exitsplashmessage." It causes a dialog box to pop up when the user tries to navigate away from the page. Don't do this. Ever. Doing so carries a sentence of being locked into every retail outlet you visit until you agree to hear a lengthy sales pitch.
3. Thou shalt not hijack system commands.
Here's a relatively innocent example. In Fruit OS, typing command-N opens a new window in Firefox. Unless, of course, I happen to have Hotmail open, in which case Hotmail hijacks that command and opens a new email within the browser tab. Don't. Just don't do this. The command key is reserved for local applications, and is not to be used by web content. Microsoft's web designers and anyone else who violates this commandment will be sentenced to have a smarmy-looking bloke follow them around who deliberately misinterprets everything they say as a solicitation for sex directed at him.