Can we create a browser extension to block all downloads within the browser? as this prevent accidental virus installations on risky websites

Had this idea, cause when I visit some websites, you can immediately tell that they will have links that download viruses / trojans - blocking websites from downloading anything - would also stop viruses from being downloaded.

Also, is it possible to create an extension - where the extension blocks websites from opening up links in new tabs - cause this would prevent a website from opening up an even more dangerous / risky website in a new tab.

> Can we create a browser extension to block all downloads within the browser?

Possibly, but ...

Not and still have a web browser left.
Downloading files from the network, especially HTML pages, is part of the primary function of web browsers.

Oh, by "all" you didn't mean web pages?
Well, before it became common to identify the Internet with the World Wide Web, there were several popular text-based services running on the Internet. Who remembers Usenet? Gopher? E-mail, of course. And there are (still) text-only web browsers, too. But combining images with text was among the killer features of the Web almost from the beginning. Few people would recognize the web in general or most of their favorite sites if browsers could not download images.

And it is extremely common for modern web pages to use stylesheets to control layout and presentation. Although these can be embedded in web pages, they are often external, separately downloaded documents. You might be surprised at how awful some sites look when their stylesheets cannot be loaded (though you may have seen an occasional example without recognizing it). Those external stylesheets are downloads, too.

And it's not too uncommon for one web page to embed another via frames or iframes. The embeded pages are downloads.

Oh, by "all" you didn't mean any of those contributing components of web pages, either?
Ok. But it is very common these days for web pages to rely on client-side scripting. These scripts can be embedded in web pages, but frequently they are in separately-downloaded files. Maybe you would include such downloads as contributing components of web pages, but at this point we're talking about stuff that could be genuinely dangerous if malicious, so this is something we do want to be careful about.

On the other hand, one of the things that such scripts often do is interact with web servers by sending their own requests and receiving (downloading) their own responses. This is fundamental to how services such as StackOverflow, Google Docs, and your web-based e-mail client work. If those downloads were prevented, then many web sites would not be functional (though perhaps that would be acceptable to some people).

Oh, by "all" you really just meant -- what? -- files that are not displayed, executed, or directly used by browsers?
Sure, that's conceivable. It would still block some pretty commonly-used capabilities, though. For example, no more installing software via your browser. I guess you'll be relying on vendors e-mailing installer programs to you. Executing programs you receive in e-mail shouldn't be any concern, right? Oh.

Overall, you can dial your risk level up or down by choosing which of these things to allow, but browsers have long had built-in security controls for doing just that. Their strictest levels are less restrictive than the strictest ones contemplated above, but generally, they are strict enough for you to really feel it when you turn them up to 11. On the other hand, built-in browser security features tend to be better contextualized than rules such as the above, so they may do a better job for the same level of restrictiveness.

> Also, is it possible to create an extension - where the extension blocks websites from opening up links in new tabs - cause this would prevent a website from opening up an even more dangerous / risky website in a new tab.

Yes, but this too is within the realm that most browsers have built-in controls for.