Question

The question is illustrated below:

<ul data-my-div='true><li></li></ul> <!-- this is surely okay -->
<ul data-my-div><li></li></ul> <!-- but is this? -->

Answer 1

Just test it! See, e.g. this fiddle. Short tests showed, that you can use in at least Firefox and Chrome.

<ul id="t1" data-my-div="true"><li></li></ul>
<ul id="t2" data-my-div><li></li></ul>


<div id="r1"></div>
<div id="r2"></div>

<script>
document.querySelector( '#r1' ).innerHTML = JSON.stringify( document.querySelector( '#t1' ).dataset );

document.querySelector( '#r2' ).innerHTML = JSON.stringify( document.querySelector( '#t2' ).dataset );

result:

{"myDiv":"true"}

{"myDiv":""}

In general, the reference explicitly allows for empty attributes here:

empty attribute syntax

Certain attributes may be specified by providing just the attribute name, with no value.