I do believe the problem is caused by the page background.
Basically, when we typed the text, highlightened the text and typed another text, the <p> will be automatically added in the html code to indicate a paragraph. Normally, if the background is default white, the <font> won't be added. As your background of the page is grey, the <font> tag is added, indicating the background is grey.
So, IMHO, in order to work around this and get rid of the <font> tag, you can do it this way:
1. type some text and highlighted the text.
2. Click
beside the BR icon to insert a paragraph(<p>), and then type some text
3. If you highlighten the text again and retype another text, the <font> tag won't be occurred.
Thank you. Hope this helps and happy using.