We are still in the infancy of webdesign. Most of the pages are deigned with a WYSIWYG-approach: What You See Is What You Get.
BUT the reality is WYSINWIG: What You See Is Not What I Get - You have no idea of the type of computer, browser and configuration I'm using to view our page. You might gain some knowledge from various parameters read from my machine. But I might e.g. select to change the size of the window at any time, without reloading the page.
This parity between reader and author is at the very core of the web. I have not yet encountered a designer which has understood this basic feature. They are trapped in the idea that they should have total control.