The following should be axiomatic:

1. Don't trust the client. Some day someone malicious will try to give you invalid input to see what will happen.
2. A significant and growing fraction of users have some form of pop-up blocker installed. Assume that any pop-up will not be seen.
3. Older users use low resolutions even on monitors that support high resolution. Any web designer who assumes that the general public has the eyesight of a 20 year old should be fired. With prejudice.
4. Users understand user interfaces differently than programmers do. What is obvious to you will never be figured out by them. If you don't have a UI specialist, then you need to learn something about good UI design because you will get it wrong.

As for the proportion of people who do not use JavaScript, it is fairly low, but far higher in any technically related website.

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