I found this in a forum post. Not sure about it's validity, but it seems legit.
It appears that Crystal Reports only supports a small subset of HTML tags and attributes.
The supported tags are:
html
body
div
(causes a paragraph break)
tr
(causes only a paragraph break; does not preserve column structure of a table)
span
font
p
(causes a paragraph break)
br
(causes a paragraph break)
h1
(causes a paragraph break, makes the font bold & twice default size)
h2
(causes a paragraph break, makes the font bold & 1.5 times default size)
h3
(causes a paragraph break, makes the font bold & 9/8 default size)
h4
(causes a paragraph break, makes the font bold)
h5
(causes a paragraph break, makes the font bold & 5/6 default size)
h6
(causes a paragraph break, makes the font bold & 5/8 default size)
center
big
(increases font size by 2 points)
small
(decreases font size by 2 points if it's 8 points or larger)
b
i
s
strike
u
The supported attributes are:
align
face
size
color
style
font-family
font-size
font-style
font-weight
Not sure about CSS. I'm pretty sure you can't do any styling beyond what you see here.