The new SharePoint Framework has a smart way to avoid conflicting CSS definitions. Therefore all style sheet classes will be post-fixed with a unique random string and converted to a JSON object. In your web part code, you can use the same class name as you used in your style sheets and the variable will be automatically replaced with the random class name string. So far the good parts of the SharePoint Framework.
In practice, this has some limitations and challenges.
When you create a new project without any framework, the default web part content in the code will be added through innerHTML.
Technically there is nothing wrong with this approach, but you might run into a problem when you add more content to the web part afterward using the same method. In this case, the previously added content will be overwritten.
To avoid this behavior, you can use jQuery and the ‘append‘ or ‘prepend‘ methods to add your content. It works well but requires an external library.
Instead of using jQuery, there is a native method in the document object model of HTML available.