SGML, the meta-language on which HTML is based, has entities ie text expansion variables bound to the content of external resources accessed via URLs or file na

SGML, the meta-language on which HTML is based, has entities ie text expansion variables bound to the content of external resources accessed via URLs or file names. So HTML itself doesn't need such mechanisms if it's understood that general markup facilities of SGML are available (which however isn't the case with browsers). XML (XInclude) attempted to establish more granular inclusion of external document fragments via XPath, but failed on the web. Even if HTML had transclusion, what would the use case look like? HTML as a markup vocabulary was introduced for casual academic publishing at CERN, reusing folklore SGML markup elements that were already widely used at the time. You can argue that HTML has been effectively stagnant while everything around it (CSS and JS) was changed ad absurdum to cater for HTML's limitations, but still in an academic setting, you'll use inline citations or block quotes with references to sources, just as HTML is providing.