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"> <