Property 'content' does not exist on type 'HTMLElement' in Typescript

TypeScript doesn't know what kind of element has this specific ID. It cannot trust the element even exists to begin with!

You need to introduce another check in order to validate your assumption:

const element: HTMLElement | null = document.getElementById('mymetatag');

if (element instanceof HTMLMetaElement) {
  element.content;
}

Update

You can also use assertion signatures.

function assert(condition: boolean, message: string): asserts condition {
  if (!condition) {
    throw new Error(message);
  }
}

Then your solution becomes:

const element: HTMLElement | null = document.getElementById('mymetatag');

assert(element instanceof HTMLMetaElement, "A <meta> element of id 'mymetatag' needs to exist in the DOM.");

element.content; // string

document.getElementById() returns an HTMLElement. But you know that your result is a meta tag, which is a type of HTMLMetaElement. Looking at the docs you can see that HTMLElement is the ancestor of HTMLMetaElement so you should downcast your element to this type. You can do this with two different syntaxes:

const metaElement = document.getElementById("mymetatag") as HTMLMetaElement;
// Or the second version:
//const metaElement = <HTMLMetaElement>document.getElementById("mymetatag");
const metaContent = metaElement.content;

See more about type assertions here.

Also, for <template id="source">, you can use HTMLTemplateElement

const source = document.querySelector('#source') as HTMLTemplateElement;
const sourceClone = source.content.cloneNode(true) as HTMLElement;

Try following:

(<HTMLMetaElement> document.getElementById("mymetatag")).content;