Stop TinyMCE from Stripping HTML & CSS code in Text mode

When you’re using the Text tab in the classic editor, TinyMCE doesn’t have access to it at all. The content you type in the textarea is saved as-is.

If you switch from Text to Visual, TinyMCE parses and sanitizes the HTML from the textarea. That usually would remove inline js (the onclick, onerror, etc.), and may sanitize some of the HTML attributes, but generally will keep inline CSS. This is a security feature.