Although one way to generate HTML is to construct a structure with objects and then walk it to produce text, an alternative approach is possible whereby our HTML-in-Lisp syntax is a macro language that compiles to code that directly generates HTML without the intermediate object. This could be quite optimized to coalesce the text where interpolation isn't happening, and do constant folding in general.
E.g. using Common Lisp as an example, this:
(html output-stream
((a href "https://example.com") "foo"))
could translate to the code:
(write-string "<a href=\"https://example.com\">foo</a>" output-stream)
which just dumps a string literal to the stream.
One issue in Common Lisp html generators is that if you want to use the backquote syntax for it, you're steered toward an implementation that just lets the backquote do its job of constructing the list, and then walk the list. The reason being that backquote expands in an implementation-defined way. If backquote expands to a macro syntax like Scheme quasiquote, you can suppress its evaluation and then walk it yourself to give it your own meaning:
(let ((url "https://example.com"))
(html output-stream
`((a href ,url) "foo")))
Here, html could intercept the quasiquote syntax, walk it itself and spit out code like:
(let ((url "https://example.com"))
(html output-stream
(write-string "<a href=\"" output-stream)
(html-write-attr url output-stream)
(write-string "\">foo</a>" output-stream)))
Historically, an example of a Common Lisp HTML formatter which walks a constructed nested list object is Tim Bradshaw's htout library. An example of an efficient generator of write-string calls is CL-WHO.
CL-WHO doesn't use backquoting for interpolation. The code template uses keywords for indicating expressions that are HTML tags. Other expressions are implicitly Lisp to be executed. Inside evaluated lisp, the htm macro switches back to HTML templating. E.g.:
(with-html-output (*http-stream*)
(:h4 "Look at the character entities generated by this example")
(loop for i from 0
for string in '("Fête" "Sørensen" "naïve" "Hühner" "Straße")
do (htm
(:p :style (conc "background-color:" (case (mod i 3)
((0) "red")
((1) "orange")
((2) "blue")))
(htm (esc string))))))
which, according to the documentation, generates code similar to:
(let ((*http-stream* *http-stream*))
(progn
nil
(write-string
"<h4>Look at the character entities generated by this example</h4>"
*http-stream*)
(loop for i from 0 for string in '("Fête" "Sørensen" "naïve" "Hühner" "Straße")
do (progn
(write-string "<p style='" *http-stream*)
(princ (conc "background-color:"
(case (mod i 3)
((0) "red")
((1) "orange")
((2) "blue")))
*http-stream*)
(write-string "'>" *http-stream*)
(progn (write-string (escape-string string) *http-stream*))
(write-string "</p>" *http-stream*)))))