
Randy H. answered 06/05/19
Certified Adobe Education Trainer and Publishing Professional
Yes, there is an Adobe program that will help you do this: Adobe Acrobat Professional has a powerful set of comparing tools you can use to highlight differences between versions of PDF files.
So the process would be to compare two InDesign files:
1) Create 2 PDFs; one of each version of your InDesign files.
2) Open Adobe Acrobat Professional. Go to your Tools, and select the Compare tool.
3) When the Compare Documents dialog box appears, choose your two PDFs for the appropriate options boxes, then click the OK button.
4) Let Acrobat Professional's Compare tools do the voodoo that it can do so well.
Now to your other questions:
No, there is no public API to access InDesign files. You'll need to have InDesign to access InDesign files, and Acrobat Professional to do your version comparisons.
Yes you can export HTML from InDesign. But it will be clunky. You'll probably do better extracting content from your InDesign files and using a dedicated web page authoring/editing program to create clean and effective HTML.
Good luck,
Randy Hagan