> what happens is the content keeps being pushed down
What happens is that the browser is re-flowing and re-painting part or all of the page, rather than updating just the area the image is known to occupy. Reflows are expensive, particularly on mobile.
Also of note is that in some circumstances (as mentioned in the link above,) the browser may have to delay rendering some parts of the page until it knows the dimensions of either the image or one of its containing elements, which means rendering the container might have to wait until it fetches enough of the image to calculate the height and width.
What happens is that the browser is re-flowing and re-painting part or all of the page, rather than updating just the area the image is known to occupy. Reflows are expensive, particularly on mobile.
https://developers.google.com/speed/docs/best-practices/rend...
Also of note is that in some circumstances (as mentioned in the link above,) the browser may have to delay rendering some parts of the page until it knows the dimensions of either the image or one of its containing elements, which means rendering the container might have to wait until it fetches enough of the image to calculate the height and width.