There is an HTML element, called "template", more information here: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template. We cannot use this element in a Kendo Editor, because it will always remove all contents from this element.
It can be reproduced in your demos here: https://demos.telerik.com/kendo-ui/editor/all-tools by following these steps:
Click the "View HTML" tool/button, then add this HTML to it:
<h1>Hello</h1> <p>This is a test</p> <template><p>This is a template</p></template>
Then open the "View HTML" tool again. You now see that the HTML has been changed to this:
<h1>Hello</h1> <p>This is a test</p> <template></template>