Can anyone explain why editing an email in the Gmail editor may look okay when you send it, but the recipient may receive it with altered editing – font, font size, broken lines, etc?

This is a general problem known since at least 2011 but never solved by Gmail.  It is so unpredictable that one should not use Gmail for serious emails.

The only safe solution is to send important documents as an attached PDF, although one workaround is to compose the email in Google Docs then cut and paste it into Gmail.

Another suggested workaround is to select the whole email message, change the font size to small and then change it back to normal, and it is claimed that it will read properly in the recipient’s email.

It is astonishing that Gmail has never sorted this in six years.