I use a Chrome/Chromium extension which makes viewing videos on Youtube a better experience. It is called "Magic actions for youtube". It is a nice extension, except it tends to be a bit noisy. I.e. when it updates it opens a page, which is annoying. The extension also adds some spammy looking "share" buttons when using the "cinema mode" which it enables (it centers the video and hides other page elements.) Anyway, to get rid of this annoying little graphic, install "userstyles" from userstyles.org and create a new style (by left clicking the userstyles button, select "find more styles for this site" and so on.)

Add this CSS:

# ma-cin-share {
  visibility: hidden !important;
}

select "applies to" "URLs on the domain" and enter "youtube.com", hit save. And enable the userstyle by checking the box on the left (under the name you gave it.)

You won't have to see the annoying element ever again. Obviously this trick can be used for any annoying HTML element which has got an identifying class name or id.