Hello Kampanart,
The HTML formatting tags are automatically inserted by the Atto editor, which does a great job. Normally, you don't have to edit the HTML code.
If you check the HTML code, you will see that the Atto editor sometimes inserts "unnecessary" tags such as <span style="font-size:1rem;">. These tags are fairly harmless, for example <span style="font-size:1rem;"> simply means to use the default
root font size.
You have to be careful if you want to clean up the HTML code and remove these tags. Make sure to delete the entire tag, that is everything inside and including the "<" and ">" characters, and the corresponding closing tag. For example, the tag <span
style="font-size:1rem;"> and the corresponding closing tag </span> can be deleted if desired.
" " is a character code which is also sometimes inserted by the Atto editor. It represents a non-breaking space. You can replace it with an ordinary space, " ", if that's what you want.
If you edit the HTML code, be sure to check the result in preview mode. Then check again the HTML code and the preview before saving. If you are editing the HTML code, in case of problem, I suggest that you always make a copy of the HTML code at least
to the clipboard before previewing and saving.
You may want to switch to the "Plain text area" editor, but I don't suggest that you do so unless you have a few years of experience working with HTML.
In short, clean up your code correctly and you should have no problem.