Your comments

Having updated to iOS 13.5.1 I do not find that the issue recurs. My htm and css fils are now tidied into one record each, and I have grown accustomed to maintaining them that way (not particularly difficult for e.g., an 889-byte style sheet). I will most assuredly repost if the issue recurs. I can give you a URL of a sample (lorem-ipsum) page if that would be … er … entertaining but I suspect the issue for now is not worth further pursuing. Thanks for your response, Mr. Blach.

[Oops I should've said "theme" writers, not "template" writers. Big difference. My bad