User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

page.html:
<ol>
<li>Item 1.</li>
<ul>
<li>Item 1a.</li>
<li>Item 1b.</li>
</ul>
<li>Item 2.</li>
<li>Item 3.</li>
</ol>

The second item in the outer list is rendered as being "item 3".

(If it's relevant, this is on Linux.)

Actual results:

Incorrect numbering of outer list.

Expected results:

It should have been item 2.

This is invalid markup, the <ul> should be inside the <li> like:

<ol>
<li>Item 1.
<ul>
<li>Item 1a.</li>
<li>Item 1b.</li>
</ul>
</li>
<li>Item 2.</li>
<li>Item 3.</li>
</ol>

But yeah, this is known fallout from making list items use CSS counters under the hood (which is how they're supposed to behave per spec).