HTML and CSS sources

https://www.w3schools.com/html/html5_intro.asp Introduction to HTML5
https://html.spec.whatwg.org/ whatwg HTML 5 specification - this document gets updated frequently
*http://www.yourhtmlsource.com/stylesheets/ Looks to be a nice section on using HTML and CSS.
http://www.inloox.com/project-management-glossary/html-software-and-resources/ HTML software and Resources. General information about HTML with links to other sources. Some links refer to older versions of HTML.
* http://www.w3schools.com/tags/default.asp Tag Reference for HTLM 5.
https://websitesetup.org/html5-cheat-sheet The cheat sheet contains a listing of HTML tags. The sheet can be downloaded.

HTML, XHTML, and CSS validaters

W3C site to validate HTML pages.
W3C site to validate CCS in web pages.
W3C site to validate links in web pages. (It took about 70 seconds to check the links in this page.)

Template page for HTML 5

http://www.brinkje.com/web/html5Template.html If you are using a standard ASCII text editor, you can avoid typing the standard parts of a web page by copying and pasting this page and then using it to start your own webpage. You should delete the link to the style page and/or the style tags if you don't use them.

Web site building for kids

http://www.ebizwebpages.com/website-builder-kids.html References that could help kids build web sites.
http://www.iwdagency.com/blog/web-development-in-the-classroom-web-site-building-for-kids/ Some comments and additional references.
https://www.smartadvocate.com/News/Blog/software-programming-and-coding-glossary-for-kids Explains a number of words used while programming.

Tutorials for HTML

http://www.tutorialrepublic.com/html-tutorial/ This seems a good HTML5 tutorial.
*http://www.yourhtmlsource.com/stylesheets/Tutorial on using CSS with HTML.
http://www.goer.org/HTML/ Seems to be a pretty good tutorial for HTML 4.01 but does not use the newer HTML5
http://www.w3schools.com/html/html5_intro.asp HTML5 Tutorial by W3schools.
*https://codeguppy.com/ Learn some Javascript by using it for graphics. These tutorials are free. Hints: Start with the After running your code tutorial and proceed though the tutorials in order. After running your code, use the symbol in the upper left corner of the run screen to go back to the tutorial.
Actually these tutorials use the p5js online editor and use the p5js extensions to Javascript.
https://praxent.com/blog/beginners-guide-to-coding-programming A Beginner's Guide to Coding and Programming.

HTML colors

http://www.brinkje.com/web/ColorMixer/ColorMixer.html Allows user to select the values for Red, Green, and Blue and see the results. It also shows the HTML named colors. It has links to color mixers for Java, JavaFX, Processing, and Processing.js. (The color mixer is written in Processing.js and only requires that JavaScript is enabled.)
https://www.w3schools.com/colors/colors_names.asp HTML 5 colors with links to other color features.
https://www.w3schools.com/colors/colors_picker.as Allows you to pick a color from a display and shows the various codes for it along with some lighter and darker colors.

CSS

*http://www.brinkje.com/web/css/CommonCSSwithStyle.html Describes and illustrates some common CSS properties
http://www.brinkje.com/web/css/CommonCSSwithoutStyle.html Describes some common CSS properties. This is the same page as above but does not use any CSS.
http://www.w3schools.com/css/demo_default.htm Demonstrates how style sheets can be used to change how a page looks without changing HTML
http://www.csszengarden.com/ Uses professionally designed style sheets to demonstrate how style sheets can be used to change how a page looks without changing HTML
*http://www.w3schools.com/css/ Tutorial on CSS
http://www.w3schools.com/cssref/default.asp CSS reference
http://www.ampsoft.net/webdesign-l/WindowsMacFonts.html Suggests "safe" font-family values that work on both Windows and Mac computers.
http://www.hostingreviewbox.com/introduction-to-cascading-style-sheets-css/ Another introduction to CSS.

Forms and Cookies

http://www.yourhtmlsource.com/forms/ Short tutorial of forms
http://www.echoecho.com/htmlforms05.htm Tutorial on forms
http://www.htmlgoodies.com/tutorials/forms/article.php/3479121 Another tutorial on forms

Download browsers

http://www.mozilla.org/en-US/firefox/new/ Download the Mozila FireFox browser.
http://www.google.com/chrome Download Google's Chrome browser.

Information about computer related topics

http://www.webopedia.com/
http://www.smartscholar.com/computer-science-guide Provides links that explain the meaning and parts of "computer science".
http://ithare.com/a-beginners-guide-to-programming-languages/ Short description of several computer languages.

Additional links

https://www.marcaria.com/ws/en/articles/learn-about-domains-and-html Many links relating to domains and HTML. Unfortunately, some of the HTML references use older versions of HTML but most modern browsers still allow obsolete versions of HTML.

Java Script

https://javascript.info/ Looks like a fairly complete tutorial.
http://www.w3schools.com/js/default.asp w3schools Javascript tutorial.
https://p5js.org/ p5.js is a JavaScript library the for creative coding, with a focus on making coding accessible and inclusive for artists, designers, educators, beginners, and anyone else>
https://codeguppy.com/ Learn some Javascript by using it for graphics. These tutorials are free. They are intended for kids, teens, artists, adults, and beginners. Hints: Start with the "Intro to coding" tutorial and proceed though the tutorials in order. After running your code, use the symbol in the upper left corner of the run screen to go back to the tutorial.
Actually these tutorials use the p5js online editor and use the p5js extensions to Javascript but don't actually say so although they hint at it in comments to parents and educators.