Pre-HTML5 we used div and ul tags to markup navigation bars and a lot of times the navigation is at the top of the HTML document. In the absence of meta description content, Google uses its own logic to determine what to use in place of the description to show in the search results. Turns out it picks the text in the navigation bar instead of the page contents.
Will using nav tags to markup navigation make Google skip the navigation? What tags will indicate to Google that the enclosed content is the main page content?
Please avoid suggesting the use description meta tag to "solve" this problem. I am more of trying to understand the behaviour and result when not using description meta tags.
<li>tag and doesn't index them because there aren't so many websites that doesn't use top navigation bars or at least left/right while they are still at the top of the markup, your best approach would be to go for the html5 navigation tags, because google will recognize it the best, also you don't need to worry.