Are you able to provide a shot of the HTML inside that editable area (click into the HTML toolbar icon)?
To my eye, it looks like you might have copied in some CSS from a source somewhere else and pasted that into the Rich Text Editor by accident. Sometimes when you copy info from a website, Google Doc or Word Doc, etc - the underlying CSS is picked up and Marketo tries to do you a solid by interpreting and rendering that CSS when you paste the text into the WYSIWYG editor (rather than into the HTML/code view -or- use the "paste as plain text" command).
There's usually a string of style="" that are about 4-5 lines long in the editor if you've run into something like, maybe best to check there and remove any style="...." attributes that you didn't add yourself. In email, this might be included in a <span></span> tag around your text or it might show up on the element (<h3>) like in the example below.
In the screenshot below, I copied some text from another section on the page by highlighting it, right clicking and choosing "copy" from the dialog box. Then I went into another editable area, clicked into, then pasted my clipboard into the Rich Text Editor (WYSIWYG - shown in the background behind the HTML Source Editor in the image). From there I clicked into the HTML icon in the toolbar to see HTML Source Editor window so that I could see the underlying issue in the code.
Junk styles that are a result of using the "paste" function inside the Rich Text Editor
Removing everything highlighted in gray here will remove the junk styles and restore the element to it's original state.
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