Reader mode in Safari (at least on iOS) renders this nicely for me automagically, and turns the grey to a nice black.

For the desktop I was going to recommend Distill Mode in Chrome (add --enable-dom-distiller to the command line/launcher, then it's available from the vertical "..." menu top right) but it didn't behave the way I remembered it. You get a nicely formatted page with all (or at least most) of the extraneous crap filtered out, but the grey remains.

Internet Explorer supposedly has a Reading Mode too, but I can't find it on my desktop.

So right. I have a grease monkey script globally to inject css to make text black. Which works well except when the background is dark of course. But I find reading light text on dark background also an effort so I usually just skip that.

On Firefox you can ask the browser to disable CSS, or even just to ignore any text color defined within CSS (thus using your browser defaults).

The text is dark gray on my screen (hex color #4d4d4d, which is 70% black). You might want to try adjusting your monitor settings.

The text is larger, and thicker than usual text in a newspaper or book, and it is just as black, or even more black (measured on a calibrated display).

You might want to change your monitor, or recalibrate it, because it seems like this is an issue with your display.

Photo: https://i.imgur.com/o5bknYs.jpg (The text on the newspaper there is #574839, while the text on the linked site is #454545. The text in the newspaper is the equivalent of 10px tall, and 1px thick, the text on the site is 16px tall and 2.5px thick)