Although impossible to know all of the errors that can be made in web coding, knowing this hand-full of common mistakes can significantly decrease the amount of trouble budding web developers run into.
Syntax
syn·tax (sin′taks′)
noun

the structure of statements in a computer language
the rules governing this structure

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Example:
A paragraph in HTML is properly written:
<p>Paragraph goes here.</p>
An incorrectly written [...]