It is very highly recommended for you to disable subreddit CSS and both RES (Reddit Enhancement Suite) and reddit moderator toolbox, if those extensions are enabled for your browser.

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This is to help get many of the catchmes like the (edit) below your own comments if you participated in a discussion and other missable elements that have gotten people outed in the past.

!!! IMPORTANT !!!

You need to DISABLE subreddit styles (as some subreddits have special CSS designed to catch leakers).
https://www.reddit.com/prefs/ -> Display Options -> []allow subreddits to show me custom themes

There are hidden, secret CSS watermarks in some subreddit styles that will reveal your username.

!!! CAUTION !!!

It is up to you to verify screenshot afterwards, CSS elements are subject to change in the event reddit's programmers modify their own website.

Rare, Moderator, & Gold elements may be missed.

Firefox has built in screenshotting:

firefox instructions; shift-f2, screenshot --clipboard --fullpage

Chrome, I personally use https://chrome.google.com/webstore/detail/awesome-screenshot-minus/bnophbnknjcjnbadhhkciahanapffepm?hl=en-US

Using this style will add a url to the top of the comments page to this stylesheet to encourage others to use it. It is commented and can be removed or changed.