Here is a list of common illegal characters that must not be included in web urls – - – and not used when creating file names that will be used on the web:

# % & * { } \ : < > ? / +

In addition to this list it is prohibited to have consecutive periods in the URL (or file name). It’s also prohibited to start or end with a space or underline, or to end with a period.

Even though it isn’t illegal to use commas or spaces in a url – it is best to not use them for consistency across different server platforms.

These rules are Web Standards.