This issue is closed but many people report it's still happening. 

https://tracker.moodle.org/browse/MDL-67360

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When adding either text or an image on the first line of the description field of an activity or resource, and then adding text on a second line which is centered, when deleting the image or text from the first line, the text is given a size of 0.9375rem and the text is now showing as left aligned, but the html shows this is centered and left aligned. In my example, I've used the URL resource, but have tested in activities and resources, all showing the same outcome.

1. Create an activity or resource,
   2. Name activity or resource,
   3. Add an image/text to the description section,
   4. Press enter to go to new line of description, enter some text 'play',
   5. Highlight text and center text,
   6. Check 'display description on course page',
   7. Click 'Save and return to course',
   8. Edit activity or resource,
   9. Click to the left of the entered text 'play' (start of the word),
   10. Backspace until image/text is gone,
   Note: The centered text 'play' is now showing to the far left of the page and is now not centered and has added in text size of 0.9375rem.
   11. Check the html of the description; <p><span style="color: rgb(51, 102, 255); text-align: center; font-size: 0.9375rem;">Play</span><br></p>
   12. Click 'Save and return to course'.

Replicated on : https://sandbox.moodledemo.net/

Also replicate in our internal systems which are on 3.5, 3.,6, and 3.7

Replicated on Classic, Boost and Snap themes.

The clean theme does not add the text sizing.