Macros, or (eval-when (:compile-toplevel) [your code here]). Caveats apply.

Vanilla macros are one option.

  (defmacro html (string)
    (some-html-parser-library:parse string))

Or you could use a function and a compiler macro to make it more of an optimization -- if the argument is known at compile-time just parse it then, otherwise wait til runtime:

  (defun html (string)
    (some-html-parser-library:parse string))

  (define-compiler-macro html (&whole form string)
    (if (constantp string)
      `',(some-html-parser-library:parse string)
      form))

Actual example, using `string-upcase` to stand in for an HTML parsing library:

  (defun html (string)
    (string-upcase string))
  
  (define-compiler-macro html (&whole form string)
    (if (constantp string)
      `',(string-upcase string)
      form))
  
  (defun foo (arg)
    (print (html arg))
    (print (html "compile-time"))
    nil)

  (foo "run-time")
  ; "RUN-TIME"
  ; "COMPILE-TIME"
  
  (disassemble 'foo)

  ; disassembly for FOO
  ; Size: 128 bytes. Origin: #x10106A207D
  ; 7D:       498B4C2460       MOV RCX, [R12+96]                ; thread.binding-stack-pointer
                                                                ; no-arg-parsing entry point
  ; 82:       48894DF8         MOV [RBP-8], RCX
  ; 86:       488D5C24F0       LEA RBX, [RSP-16]
  ; 8B:       4883EC18         SUB RSP, 24
  ; 8F:       488B55F0         MOV RDX, [RBP-16]
  ; 93:       488B0576FFFFFF   MOV RAX, [RIP-138]               ; #<SB-KERNEL:FDEFN HTML>
  ; 9A:       B902000000       MOV ECX, 2
  ; 9F:       48892B           MOV [RBX], RBP
  ; A2:       488BEB           MOV RBP, RBX
  ; A5:       FF5009           CALL QWORD PTR [RAX+9]
  ; A8:       480F42E3         CMOVB RSP, RBX
  ; AC:       488D5C24F0       LEA RBX, [RSP-16]
  ; B1:       4883EC18         SUB RSP, 24
  ; B5:       488B055CFFFFFF   MOV RAX, [RIP-164]               ; #<SB-KERNEL:FDEFN COMMON-LISP:PRINT>
  ; BC:       B902000000       MOV ECX, 2
  ; C1:       48892B           MOV [RBX], RBP
  ; C4:       488BEB           MOV RBP, RBX
  ; C7:       FF5009           CALL QWORD PTR [RAX+9]
  ; CA:       488D5C24F0       LEA RBX, [RSP-16]
  ; CF:       4883EC18         SUB RSP, 24
  ; D3:       488B1546FFFFFF   MOV RDX, [RIP-186]               ; "COMPILE-TIME"
  ; DA:       488B0537FFFFFF   MOV RAX, [RIP-201]               ; #<SB-KERNEL:FDEFN COMMON-LISP:PRINT>
  ; E1:       B902000000       MOV ECX, 2
  ; E6:       48892B           MOV [RBX], RBP
  ; E9:       488BEB           MOV RBP, RBX
  ; EC:       FF5009           CALL QWORD PTR [RAX+9]
  ; EF:       BA17001020       MOV EDX, 537919511
  ; F4:       488BE5           MOV RSP, RBP
  ; F7:       F8               CLC
  ; F8:       5D               POP RBP
  ; F9:       C3               RET
  ; FA:       0F0B10           BREAK 16                         ; Invalid argument count trap

`html` is only called once, to handle the call that isn't known at compile time.