Include files inside HTML, using a new custom tag, <include component="path">.

including.ts

// Fix Array.from not being defined.
interface ArrayConstructor {
    from<T, U>(arrayLike: ArrayLike<T>, mapfn: (v: T, k: number) => U, thisArg?: any): Array<U>;
    from<T>(arrayLike: ArrayLike<T>): Array<T>;
}

window.addEventListener("load", () => {
    const needIncluding = Array.from(document.getElementsByTagName("include"));
    for (const include of needIncluding) {
        includeFile(include.getAttribute("component"), include);
    }
});

const includeFile = async (fileName: string, target: Element) => {
    const content = await getIncludeContent(fileName);
    target.replaceWith(content);
    return true;
}

const getIncludeContent = async (fileName: string) => {
    const result = await (await fetch(fileName)).text();
    const parser = new DOMParser();
    return parser.parseFromString(result, "text/html").body;
}