What is the key disadvantage of absolute positioning for responsive web design?

An absolutely positioned element is pulled out of the normal document flow, so surrounding content ignores it — which breaks layouts that need to adapt to screen size.

Normally, block elements stack one after another and push each other around: the "document flow." When you set position: absolute, the element is lifted out of that flow and placed at coordinates you specify (via top, left, etc.). The problem for responsive design — designs that reflow gracefully on phones, tablets, and desktops — is that the rest of the page now behaves as if the element were not there:

• Other elements don't reserve any space for it, so they can end up overlapping it.
• The layout won't reflow when content above changes height.
• Fixed pixel coordinates don't adapt to different viewport sizes.

So when is absolute positioning still the right choice? When nothing else works: positioning something with JavaScript, or deliberately overlapping an element on top of others — think of an always-visible chat button or a tooltip that floats over the content.