Question

Note the li with child ul is display:inline
– code example as follows

<style type="text/css">
ul.nav_main li { display: inline }
ul.nav_submenu li { display: block }
</style>

<ul class="nav_main">
    <li>Item 1
        <ul class="nav_submenu">
            <li>Option 1</li>
            <li>Option 2</li>
        </ul>
    </li>
</ul>

Answer 1

CSS does not affect validity of HTML source. It may however cause unpredictable results, as the browser try their best to place a UL inside of an inline element (even though it’s valid!)

Any way, you’re always welcome to check it yourself