The essential web development language for styling web pages is CSS (Cascading Style Sheets). CSS works alongside HTML to provide the layout, colors, fonts, and overall visual appearance of a website. It is critical for making web content aesthetically pleasing and improving user experience. By separating content from design, CSS allows developers to maintain and update the look of multiple web pages simultaneously. Mastery of CSS is essential for front-end developers to create responsive, visually engaging, and user-friendly web interfaces.