I use (a old version of) Firefox and can select "View > Page Style > No Style" to disable CSS, and this works OK for me (it is better than some web pages, where this does not work very good, but this one it works good).
I do not know what criteria are needed for the reader mode in Chrome. (The HTML code looks OK to me?)
I do not know what criteria are needed for the reader mode in Chrome. (The HTML code looks OK to me?)