Nested Lists - li is a child of ul!
Keep in mind that the only allowed child of a ul element
is an li element.  So in a nested list, the nested  ul needs to contained within the li:
| Correct | Incorrect ul must be within li | 
|---|---|
| Example in GistView ExampleValidate Example  | Example in GistView ExampleValidate Example  | 
Sometimes this is confusing, since in the "HTML" form of HTML5, the end tags are options for li, so while it may appear that you can have a ul as a child of ul, you really can't -- there's an implied end-tag there.
| Correct HTML serialization | Correct XML serialization | 
|---|---|
| Example in GistView ExampleValidate Example  | Example in GistView ExampleValidate Example  |