Web application: How to open a file on a "new window" on a web application (like google mail does opening attached files)?

I created a simple web application. Users have to choose some parameters to get start it and to have at the end an svg image to download. But, in the download page, before the user download the image, I would like he could have a preview of the image. I know that there are some other questions about it, but I didn't find what I am looking for. Because I would like my preview opens in a window but without leave the download page. That one would be the background of the "preview" window.

To give you an example I would like to have the same effect of google mail when you have an e-mail with an attached file e you just click on the file and a "transparent" window is opened but you can close it and come back to the underlying page.

How to do this with html and css?

many thanks