REFER TO 3.2.1 "Data Characters"

REQUIRED ASSERT: Any sequence of characters that do not constitute markup are mapped directly to strings of data characters.

abc < def <,def <"def

abc abc def

REQUIRED ASSERT: An ampersand is only recognized as markup when it is followed by a letter or a `#' and a digit:

abc & def &,xx &.xx &

abc &# def &#&

ESSENTIAL ASSERT: A useful technique for translating plain text to HTML is to replace each '<', '&', and '>' by an entity reference or numeric character reference

looks like markup: <A href="foo"> &lt;