Some things to watch out for:
1. Make sure all your tags are lower case.
2. Each <p> needs an </p>.
3. Every <li> needs a </li>.
4. End all empty elements. For example, break tags can be written <br></br> or <br/>.
5. Code all special ASCII characters. For example, use & instead of &, even in URLs.
6. Make sure all your attributes, including href URLs are enclosed by quotation marks.
7. Check to see that nested tags open and close in the same, correct order.
Here’s a checklist you might find useful: Common errors