Recently we have uploaded an aspx page inside the site assets document library in SharePoint Online site. When we click on the aspx page, instead of opening it in the browser, it asks to save the document.
Our requirement is the file should be open in the browser, it should not ask to save the file.
We have also checked the “Opening Documents in the Browser” settings in the library settings which was already set as “Use the server default (Open in the browser)”. We can check in library settings -> Advanced settings and on the advanced settings page you can see “Opening Documents in the Browser”.
You can see the figure below:

But still, the aspx page was asking to save instead of opening in the browser.
When we upload the aspx page, we use the drag-and-drop feature of SharePoint to upload the aspx page. Then, to resolve the issue, we click on the Upload button and then browse the file, and then upload the document.
Check out the below pics of how we can upload the document instead of drag and drop, and it worked well.

I hope you got an idea of SharePoint Online document library aspx pages asking to save not opening in browser.
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I am Bijay a Microsoft MVP (10 times – My MVP Profile) in SharePoint and have more than 17 years of expertise in SharePoint Online Office 365, SharePoint subscription edition, and SharePoint 2019/2016/2013. Currently working in my own venture TSInfo Technologies a SharePoint development, consulting, and training company. I also run the popular SharePoint website EnjoySharePoint.com
For me I tried this on the sharepoint online, and did the upload option for the non-working site. Still it does not work, always downloads the aspx file.