Before starting using Bootstrap 4, let's learn about a few common patterns
Simply put, if we don't use Bootstrap 4 but use hand-carved methods to write the web page, we have to write the HTML content ourselves (nonsense), and we have to set the CSS one by one (another nonsense). However, if we use Bootstrap 4, he has already written many commonly used CSS for us in advance. As long as we are familiar with the Bootstrap 4 files and know which class name the CSS he prewrites is used in, we just need to apply it directly to the label.