Turns out Microsoft Edge, unlike Safari, can still open local html files. Discovered in this answer. To wit:

  1. Install Microsoft Edge from the App Store
  2. Open Files (or whatever file browsing app you like).
  3. Open the file, then tap the Share button to send to another app.
  4. Scroll across to "More...", choose Edge, and voila!
Answer from Heath Raftery on Stack Overflow
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Reddit
reddit.com › r/ios › ipad: how do you open a local or network html file in safari (or other browser)?
r/ios on Reddit: iPad: How do you open a local or network HTML file in Safari (or other browser)?
July 14, 2023 -

I have an HTML file in iCloud that I want to open in an iPad browser. I don't see any options in Safari. Is there a URL address bar syntax like "File///"? Browsers are not available in "share" from Apple "Files".

(more info below...)

When I click on the file, the Readdle browser is pre-empting Safari. If I could figure out how to open the file directly from Safari or Brave, my problems would be solved. The Readdle browser doesn't show an "address" for the file when it's opened, so I can't copy it. I can't just delete Readdle without moving a bunch of files around etc. (I used Readdle before "Files" was added.)

I have "a-Shell" installed, but it won't let me access system files.

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MacPowerUsers
talk.macpowerusers.com › software
In Search of an app on iPad to show HTML file directly from Shortcuts - Software - MPU Talk
March 12, 2022 - I am searching for an iPadOS app that can be called via Shortcuts to read HTML files stored locally or in iCloud. I have an HTML file that I generate as output from Curio. It shows a status shelf report. I store this in iCloud so that I can view it off line. I have a Shortcut to open the HTML.
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Apple Community
discussions.apple.com › thread › 255549790
How can I open local HTML files on my iPh… - Apple Community
However, the saved document probably ... the file paths are still trying to find the document in the context of the web server. Instead, you can save the websites to your Reading List. To enable offline reading of items in the list, go to Settings > Safari, then turn on Automatically Save Offline (below Reading List). Save webpages to read later in Safari on iPhone - Apple Support ... Are you still able to view website HTML in Safari on iPad...
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Microsoft Learn
learn.microsoft.com › en-us › answers › questions › 2007275 › how-can-i-make-edge-open-a-local-html-file-on-an-i
How can I make Edge open a local HTML file on an iPad? - Microsoft Q&A
I have read elsewhere that Edge can open local HTML files stored on an iPad with full functionality (CSS, jQuery, etc.). Other browsers, including Apple's own Safari, will not do this. The only example I've read re using Edge for this discusses opening an HTML file attached to an email, which ...
Find elsewhere
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Apple Community
discussions.apple.com › thread › 256102223
How to open local html files in safari on… - Apple Community
Due to evolving security reasons, you can no longer open an HTML file directly with Safari (or Firefox) in iOS 18.5.
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JustAnswer
justanswer.com › computer-programming › neemi-trying-view-html-docjuemnt-iphone-working.html
How to Open and View HTML Files on iPhone - Expert Q&A
Download the HTML file you wish to open. Navigate to the file manager. Press and hold the HTML file for a few seconds. A pop-up will appear; select “open with.” Open the file using Safari.
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Quora
quora.com › How-do-you-open-an-HTML-file-in-Safari
How to open an HTML file in Safari - Quora
Depending on what application is set as the default for a .htm or .html file type. A drag and drop to Safari in the Dock would work. One can also control click or right click on a file, and choose Open With and select Safari
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Apple Community
discussions.apple.com › thread › 252471413
How to open .htm and .html files in Safar… - Apple Community
I have HTML file and it's data on my Ipad. There is no such option like open with ....after long pressing of HTML file. ... They are both HTML files. Every web broswer can open them. Use Safari.
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YouTube
youtube.com › watch
Apple: View local HTML files using web browser on iPad - YouTube
View local HTML files using web browser on iPad Hey guys! Hopefully you found a solution that helped you! The Content is licensed under (https://meta.stackex...
Published   March 21, 2024
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Apple Community
discussions.apple.com › thread › 254775223
How do I launch an html file via safari? - Apple Community
You can also Control-click the file, then choose Get Info. In the Info window, click the arrow next to “Open with.”
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Stack Overflow
stackoverflow.com › questions › 53471215 › viewing-html-from-a-mac-on-an-iphone-locally
xcode - Viewing .html from a Mac on an iphone locally? - Stack Overflow
Go the the URL listed in the Web Sharing panel from your iPhone, while on the home network, and open the html file. It will load in Mobile Safari.
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Apple Community
discussions.apple.com › thread › 2661059
How to use iPad's Safari to open HTML fil… - Apple Community
Nov 25, 2010 4:21 AM in response to John H. Lincoln · No, mobile Safari can't open locally stored HTML files, as for security reasons iOS devices do not have an open file system like your Mac has. I think that there are third-party Apps that can do this (such as GoodReader maybe?). You have ...
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MacPowerUsers
talk.macpowerusers.com › software
Issue Showing a Local HTML file in Safari iOS 14 - Software - MPU Talk
October 17, 2020 - I have an HTML file that I store in an iCloud folder. Under the previous iPadOS (13), I could run a shortcut to display this html file in Safari. Under iOS 14, I cannot do this. As near as I can tell by reviews on the Apple forum and StackExchange, this issues is related to one of two possible ...
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NerdsChalk
nerdschalk.com › open-html-files-iphone
How to Open HTML Files on iPhone
January 20, 2024 - The Files app will now open the ... the same page as the HTML file directly on Safari, you can load the embedded links on the page and open it on the Safari app....