[tannhaeuser]

HTML was invented as an SGML vocabulary, and SGML does have those inclusion mechanisms (called entities), though browsers basically never implemented those. To this date, self-acclaimed web folks ridicule XML (a simplified subset of SGML originalky invented as the base for additional vocabularies on the web, of which SVG and MathMl made it) for daring to incorporate something as basic as text variables/entities ie the "billion laughs atrack" (SGML could already limit the recursive entity expansion depth).