The grey is the div below. It is widthed 40em and will grow and shrink with the size of the text when looked at in browsers that have the option for Zoom Text Only. The size of the text is operated in some browsers by the controls in the View menu, often Command or Control + or -. The image, as it happens, has a native resolution of 789 pixels width, plenty big enough for most viewers and lots of scope for sharp reduction according to text sizes and/or viewport sizes.
Above is sized 95% of the width of the div.
Above is sized 38em.
Above is sized 250px.