Pizza v ingredients
If a page in a book is like a pizza, then a page on the web is the ingredients for the pizza plus the recipe to make it.
The browser makes up the web page from the ingredients (content) using the recipe (instructions for structure and presentation).
In most cases you want the browser to follow the same recipe.
In some cases you may want it to follow different recipe (eg for printing, text-only version).
Different editions for different media are easy to do if you separate content from structure and presentation.
Content is yours to create, edit and manage The designer's job is to provide an appropriate presentation for the structure you give your content.
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