HTML (Hyper Text Markup Language)

The 'HTML' part contains all the content, organized into a logical structure.

This is the part that an author might be most concerned with: the words, chapter headings, figures, diagrams, etc.

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CSS (Cascading Style Sheets)

The 'CSS' part (version 3 being current) is all about the presentation or style of the page.

This is to set fonts, sizes, indentations and whatever else may apply to what webpage looks like.