This is very likely coming from some app. It's called data attribute and data attributes per se are harmless, it won't slow down your website or anything, so I wouldn't worry about it.
Thank you for your reply.
Is there a way we can know for sure where it is coming from?
Is it normal that the html is duplicated rather than having only the data attributes? And can you tell if the data attributes are harmless in my case according to the screenshots?
Hello, I have it on my store and I only have the apps Dsers and SEO King. Therefore I think it comes from SEO King
Yes, it is a "data attribute" element. Will it slow your site down? No. However, it could cause your HTML to go awry! I believe that has more to do with how Shopify's WYSIWYG editor handles the formatting.
I know this is an old topic, but I see the data-mce-fragment get added to the HTML when someone is copying/pasting text from Microsoft Office apps to Shopify. It's very possible that other apps work the same way.
Many times a person will write the product copy in some other document then send to someone else to be entered in Shopify. Then the person entering the text into Shopify does a quick copy/paste to load the data, and those tags end up coming across but most people don't even notice because they're hidden in the file the text was sent in, and the default Shopify editor doesn't show them either. You'd have to be looking at the HTML source to see them.
Yes!! It happens if you are copying from Microsoft Word. We can either delete it after copying and pasting it. Or we can copy the text on the code part and manually format it (my way, since the code looks cleaner).