To create and edit a website, you need a text editor. Text editors create and modify unformatted text files. Other formats, like RTF, let you to add formatting, like bold or underline. Those formats are not suitable for writing web pages. You should put some thought into which text editor you use, since you'll be working with it extensively while you're building the website.
But testing gets complicated because some browsers only run on certain operating systems. Apple Safari runs on iOS and Mac OS, while Internet Explorer runs only on Windows. It's best to take advantage of services like Browsershots or Browserstack. Browsershots furnishes screenshots of your website as it will look in various browsers. Browserstack actually gives you full remote access to virtual machines so you can test your site in the most common environments. Alternatively, you can set up your own virtual machine, but that takes some expertise. (If you're going that route, Microsoft has some tools for developers including a ready-to-use virtual machine at modern.ie.) how to build a website