Internal CSS in frontend development is used for styling specific web pages without referencing an external stylesheet. It is a way to implement CSS rules on page elements in an efficient manner. Internal CSS is taught in a frontend development course in Madurai to improve web design skills. It facilitates the development of attractive, responsive layouts with appropriate color schemes, fonts, and spacing. Learning internal CSS in addition to HTML and JavaScript provides a solid foundation in web development, which is an important skill for potential developers.