When I surfing the internet I used to find the web is overflowing with sites that use horribly invalid, broken, and inaccessible markup. Even sites built by people who have been in the web business for many years, and who really should know better, are full of problems that shouldn’t be there. The reason? Ignorance, laziness, lack of time, bad tools, you name it. Yes, I’ve been guilty of making many mistakes myself through the years. However, I do my best to learn, and avoid making the same mistakes over and over again.

Here’s a list, in no particular order, of some of the most common mistakes that even experienced web professionals tend to make:

DOCTYPE confusion
Completely missing, incorrect, or in the wrong place. I have seen HTML 4.0 Transitional used in documents containing XHTML markup as well as in <frameset> documents, DOCTYPE declarations appearing after the opening <html> tag, and incomplete DOCTYPES.

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