The importcss plugin adds the ability to automatically import CSS classes from the CSS file specified in the content_css configuration setting.
By default selectors like ".my-class", ".my-class1.my-class2" and "p.my-class" get imported as format rules.
tinymce.init({
selector: 'textarea', // change this value according to your HTML
plugins: 'importcss',
content_css: '/my-styles.css'
});These settings affect the execution of the importcss plugin, specifically the way that these operations are handled.