parent, css menu ul li parent child problem

Hello to everybode!
I found on internet a custom select menu, made with HTML and CSS.
The problem is that when I am using it inside a <nav> tag it is looking and behaving different than when it is alone on the webpage.
Here is the original from internet:
HTML:

<form>
 
        <ul class="select">
            <li>
                <input class="select_close" type="radio" name="awesomeness" id="awesomeness-close" value=""/>
                <span class="select_label select_label-placeholder">#</span>
            </li>