One of the easiest ways to test your HTML code for errors is to use an online validator tool, such as the W3C Markup Validation Service. This tool checks your code against the HTML standards and reports any errors or warnings. You can either enter your URL, upload your file, or paste your code directly into the tool.

I recommend learning HTML and CSS on freeCodeCamp and Codecademy. First, learn it on freeCodeCamp next if you want to master HTML and CSS then take Codecademy's course, like this you can master HTML and CSS. Also if you want to make your website more modern, cooler, and stylish then learn Bootstrap and SASS.

Fixing CSS Errors involves identifying the specific issues in the code and making the necessary corrections. This may include debugging the code, updating outdated properties, and resolving conflicts with other elements on the website.

The final step in debugging your web development is to use the extension. To do this, open an HTML file in Visual Studio Code and start editing your code. You'll notice errors and warnings highlighted in different colors and symbols, which can be viewed by hovering over or clicking on them.