First check the colour profile embedded in your images when exported to the web. Are they converted (if necessary) to sRGB profile rather than whatever profile you are using during Post Processing.
MOST, but not all browsers, will assume sRGB profile in web published images and other browsers which are colour managed to cope with other embedded profiles WILL all be capable of dealing with sRGB images too.
So the recommendation is to use sRGB for the web.
Unsuitable profiled images will not look as they are intended if they are viewed in in unmanaged browsers. That is the commonest reason for surprising web appearance.
Another reason is when the viewing monitor is set for less than full colour. 256 colour setting will result in posterisation and banding along with a very restricted palette.