How can I run the html pages in the browser, because when I add a page with html extension, it downloads as if it were a document.
Thanks.
As a workaround try to rename yourhtmlpage.htm to yourhtmlpage.aspx then it will be displaying in the browser.
Content Editor Web Part
and paste the URL to file in document library. In case you want to render directly, change the extension to aspx. For modern page, I am searching right now what are the possible ways.