CSS Full form is Cascade Style Sheet. It is used for decorating the HTML document. The flex property in CSS is the combination of flex-grow, flex-shrink, and flex-basis property. Without CSS, HTML element should be visible in front of the browser of the user. It is used to set the length of flexible items. The flex property is much responsive and mobile-friendly. It is easy to positioning child elements and the main container. The margin doesn't collapse with the content margins.