When working with HTML and CSS is www.w3schools.com I use. This site offers different tutorials (for example, HTML5, CSS3, and Jscript), extensive reference information with search options and examples. This is a must. Often I this tutorial open on a second screen so it is easy to "borrow" (copy and paste) pieces of code.
The next option is www.html5andcss3.org. A similar site with tutorials for HTML5 and CSS3.
Another option that I can recommend is html.net. Here you will find tutorials for HTML(5), CSS(3) and JavaScript.The last option to keep is the Wiki of W3C for details on the construction of HTML and CSS code www.w3.org/wiki/Main_Page.
Many roads lead to Rome (as we say in Dutch). Using Google and you'll find a lot of editors, each with its own characteristics and peculiarities. For building the Website I mainly use BlueGriffon®. This choice was based on the "What you see is What you get" option, on the built-in code-checker and on all kind of "internal" script libraries. To maintain my style sheets (CSS) was NotePad++® more practical (in relation to Bluegriffon). This program has syntax checkers for both HTML and CSS. For the "Quick and Dirty" work is the simple NotePad® of MS-Windows® my favorite instrument.
For testing of my Website, it is essential to test with multiple viewers. Not always they react in the same way. They support a certain functionality or not. On my PC I use Edge®, Internet Explorer®, Firefox® and Opera®, on my cell phone I use Firefox® and Chrome® and Safari® on my Ipad.
On the website of w3.org it is possible to check the code for CSS and HTML. This is useful to find errors or none existing properties. For CSS it is jigsaw.w3.org/css-validator/ and for HTML it is validator.w3.org.