complication is that XML can be more easily checked and automatically
refactored than plain HTML, which eases maintenance.
+Watch out for the ampersand "&" and angle brackets "<" and ">". They will
+render your XHTML invalid, and must be replaced by their named entities
+"&", "<", and ">".
+
3. Custom classes
=================
paragraphs or lists, enclose them in a <div>..</div>. Wherever possible,
create semantic classes rather than visual ones.
-.left: make an element float left in the surrounding paragraph.
-.right: make an element float right in the surrounding paragraph.
+.left:
+make an element float left in the surrounding paragraph.
+
+.right:
+make an element float right in the surrounding paragraph.
+
+.note:
+use for important notes that should be visually distinct from the
+normal text flow, or asides. Currently rendered in a gray box.
+
+.warning:
+use for potentially dangerous situations involving data loss, malfunction,
+or sound quality issues. Currently rendered in a red box.
4. Element use