What you see might be what you get, but it's not what I get, for reasons that are innate to the web.

My mother still uses an iPhone SE with a 4.7" screen, while I use a gargantuan phablet. My grandmother has poor eyesight and runs her browser on a 720p screen at 175% zoom, while I have a large high-resolution monitor and normally open browser windows to 1280x2160 portrait. My monitor is color-calibrated and extremely bright during the day, but after sunset it's deep amber and very dim.

If you think that WYSIWYG for the web is desirable, you haven't understood the web.