This is what helped me. For adding a block of html code from b.html to a.html, this should go into the head tag of a.html:
<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
Then in the body tag, a container is made with an unique id and a javascript block to load the b.html into the container, as follows:
<div id="b-placeholder">
</div>
<script>
$(function(){
$("#b-placeholder").load("b.html");
});
</script>