Pure HTML is specified as a symbolic expression and output in HTML. For example:
html_document("file", "Title", [
html_h1("PDEs"),
"The equation", {nabla*u = f(u)},
"is the", html_i("Poison equation"),
"and", html_center_math({d u_vec / d t = nabla .* u_vec}),
"is an advection equation."
]).
This creates an HTML file
The advantage is that routines can be written that create HTML or XML output represented as symbolic expressions (cf. the DOM of XML).
This keeps HTML/XML API very simple.