Function lowerHtmlImports

Lowers an HTML import into a module-level <template> that is parsed once, so every component instance clones it instead of re-parsing markup — the markup analog of the constructable-stylesheet lowering in lowerCssImports:

import tpl from './card.html';
// …in the constructor:
root.appendChild(tpl.content.cloneNode(true));
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becomes, in the emitted module:

const tpl = (() => { const t = document.createElement("template"); t.innerHTML = "…html…"; return t; })();
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The HTML file is read at compile time and inlined as a JS string literal (it is author-controlled source, like the component itself). Imports with no default binding, or whose HTML file can't be read, are left as-is.