execCommand("indent") conflicts with the content model of <ul>/<ol>

Consider the following tree:

<ul>
<li>first</li>
<li>second</li>
</ul>

Here, execCommand("indent") on the second list item generates the following tree:

<ul>
<li>first</li>
<ul><li>second</li></ul>
</ul>

The resulting tree inserts a <ul> inside another <ul>, conflicting with HTML:

Content model:
Zero or more li and script-supporting elements.

I guess we have two options:

• Change the indentation behavior: I don't think we want to change the behavior that has been there for decades, though. The previous author also preferred the current behavior because of algorithm simplicity.
• Change HTML. This was once rejected but IMO is the way to go for web compatibility.

Pinging @annevk for the HTML part.

Start with the example trees in this issue, then read the linked HTML ul content model and the execCommand indentation algorithm. A resolution would require deciding whether to change indentation behavior or the HTML model, followed by updating the relevant specification text; no file or test is identified.