HTML import is a tool to include HTML documents and web components from external sources without using AJAX (Ajax (programming)) requests and similar methods - w3.org 
The syntax for doing an HTML import looks like this:
<link rel="import" href="mydocument.html">
To avoid loading and executing scripts twice that are introduced through imports, HTML import skips loading and executing files that have previously been loaded -
webcomponents.org
Mozilla Firefox (Firefox) has no support for HTML import and does not intend to implement it -
mozilla.org
HTML Import is also related to an early idea in the development of hypertext called Transclusion, where all or part of an document is inserted into another by reference.