The case is mentioned here.
HTML Pages
Sometimes it is advantageous to use HTML pages of .html extension.
Example: Fully immersive pages with No SharePoint branding requirements
The Problem
Opening an .htm or .html page in SharePoint will actually download it.
![download]()
Clicking on the file will lead to the download prompt.
![prompt]()
The Solution
We need to change the following setting in Central Administration > Web Application > General Settings.
![setting]()
Change the Browser File Handling to Permissive.
![Permissive]()
Save changes by clicking the OK button.
Testing
Now click the .html file again and you will see the file get opened in browser.
![browser]()
This confirms our setting was applied. Now we can create Fully Immersive Pages with .html extension.
Note:
Please note that the Browser Handling Setting is protecting the user from opening any Script content types. By default this option is set to strict mode. Please consult with your security team too before changing this setting.
References
http://bit.ly/1RxzPwH
Summary: In this post we have explored a common scenario while using HTML pages in SharePoint.