As HTML settles into its 30's we need to acknowledge the work that has been done since Sir Tim Berners-Lee invented HTML in 1991 by Sir Tim himself, Dave Raggert, the W3C and WHATWG in standardising HTML in its various flavours to arrive at HTML 5. HTML gives a standardised structural framework within which a Web Designer my develop code that may be understood with any collaborators and displayed on a website as the developer intended.
Cascading Style Sheets allow the Web Designer to develop a toolkit that will easily and quickly allow the look of the Website to be applied across all similar aspects. In my mind it is a highly powerful extension to what we do when we create a paragraph type or heading type in a Word Processor - it allows the Web Developer to create a 'look and feel' for a heading
h1 for example, that as soon as the Web Developer adds a h1 that look and feel wrt text size, text color, background, typeface will automatically be added to that h1.