Formatting mysteriously keeps changing

Registration date
Thursday September 30, 2021
Last seen
September 30, 2021

Yahoo keeps changing the size format of my text without my permission. For ex., the entire body of an email is supposed to be written in Modern medium. Instead, a couple of paragraphs are Modern small. This gives my emails a very sloppy look.

The formatting change happens even though I do not copy-paste text from Word or other websites. I avoid using the "Switch to plain text mode" because reverting to plain text removes URLs etc. which is not handy.

I use Chrome and the same problem happens with other browsers. I sure would appreciate some help! Thanks, Margaret