regex101.com: This is my favorite regex site and it is used to test regular expressions. You can have your own account, save your regex in your own private repo, or in a public library. It also explains how each part of the regex works and colors each character to how it was found with the regex. Worth a look.
regexr.com is another online site to test regular expressions.
Color picker: Sometimes web developers need to find that perfect color so a color picker is essential. This one has various ways and color models to pick a color and convert it to #rrggbb format.
How do you make a color palette to match a specific image? With this site. Upload an image and it generates a CSS color palette to make your website all purdy. It generates light, medium, and dark color palettes.
How do you generate a CSS gradient? With this tool. CSS supports gradients, which can make your area look like it has a horizon, or just make an area pop with color.
And don't forget Markdown. There are several variants of Markdown but it allows you to write and format text without all that pesky HTML getting in the way and making things hard to read. Then the Markdown is converted to HTML, PDF or another format, and then you do what you want with that formatted output.
Dillinger.io is an online markdown editor that shows a formatted preview which shows you what your output will look like. You can save your items to Google Drive, Dropbox, One drive and Github. You can also export the file to Markdown, HTML, PDF and styled HTML (I assume this creates an inline style sheet for the HTML page). You can also import from Dropbox, Github, Google Drive, One Drive, and a local file from your PC. Dillinger is open source, stored on Github, and is free to install on your own server.
Pandoc converts many text formats into other text file formats like Markdown to HTML or Markdown to an EPUB book. It has a ton of options so read carefully. And it's free. For Windows, Mac, Linux and BSD.