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- Put all tag names and attribute names in lower case.
- Make sure every start-tag has an end-tag. This rule does not apply to the HTML empty tags, namely basefont, br, area, link, img, param, hr, input, col, frame, and isindex. (If you don't know what some of these are, don't worry about it).
- Replace the > at the end of an empty tag with the three-character sequence " />".
- Make sure all start-tags and end-tags are properly nested.
- Make sure all attribute values are in quotation marks, either single or double.
- Make sure attributes like "checked", that don't have values, are written "checked='checked'".
- Any & and < characters, even in scripts or stylesheets, must be replaced by & and < respectively.
- Don't wrap scripts in comment markers (<!-- ... -->).
- Make sure you use the semicolon after an entity reference like á.