how to secure open securedoc.html?

I got call from revenue and asked them confirmation that’s it’s call actually from them.

They sent me email (I check Gmail original and it’s actually from revenu) with securedoc.html

I need to open this attachment to read secure message.

But I’m not sure if it’s secure to open html from your local drive. I read the the following discussion:

The browser has a security sandbox. When you visit a site, that sandbox only permits code on that site to access content on the same domain. If you run html from your machine, then the sandbox now includes your filesystem – and thus can be used to access anything the user can.

Can I open html secure? Maybe in incognito mode?

I saw similar question, but it’s about email from unfamiliar sources, in my case I know source.