Remove unneeded learning-area refs

[chrisdavidmills]

Description

This PR is part of a recent project to clean up the Learn-related repos by removing all files from learning-area that are no longer referenced here, hopefully saving us a lot of maintenance work in the future.

This PR focuses just on the accessibility and css directories; it has gotten big enough already. I've:

• Removed all references to learning-area that are no longer needed here.
• Created some new live samples, thereby further increasing the number of learning-area files and references that can be removed.
• Done a few other bits of assorted cleanup

The accompanying learning-area PR is at mdn/learning-area#848. These two PRs need to be merged simultaneously to avoid breakage.

Motivation

Additional details

Related issues and pull requests

[github-actions]

Preview URLs (14 pages)

• /en-US/docs/Learn_web_development/Core/Accessibility/Accessibility_troubleshooting
• /en-US/docs/Learn_web_development/Core/Accessibility/HTML
• /en-US/docs/Learn_web_development/Core/Accessibility/Mobile
• /en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics
• /en-US/docs/Learn_web_development/Core/CSS_layout/Legacy_Layout_Methods
• /en-US/docs/Learn_web_development/Core/CSS_layout/Multiple-column_Layout
• /en-US/docs/Learn_web_development/Core/CSS_layout/Positioning
• /en-US/docs/Learn_web_development/Core/Styling_basics/Advanced_styling_effects
• /en-US/docs/Learn_web_development/Core/Styling_basics/Backgrounds_and_borders
• /en-US/docs/Learn_web_development/Core/Styling_basics/Tables
• /en-US/docs/Learn_web_development/Core/Text_styling/Styling_lists
• /en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts
• /en-US/docs/MDN/Writing_guidelines/Page_structures/Code_examples
• /en-US/docs/Web/HTML/Reference/Elements/input/search

External URLs (8)

URL: /en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics
Title: WAI-ARIA basics

• https://mdn.github.io/learning-area/accessibility/aria/form-validation-checkbox-disabled.html (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn_web_development/Core/CSS_layout/Legacy_Layout_Methods
Title: Legacy layout methods

• 🚨 http://getskeleton.com/ (1 time) (Note! This may be a new URL 👀)
• https://mdn.github.io/learning-area/css/css-layout/legacy/html-skeleton-finished.html (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn_web_development/Core/Styling_basics/Advanced_styling_effects
Title: Advanced styling effects

• https://mdn.github.io/learning-area/css/styling-boxes/advanced_box_effects/filters.html (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts
Title: Web fonts

• https://mdn.github.io/learning-area/css/web-fonts/google-font.html (1 time) (Note! This may be a new URL 👀)
• https://mdn.github.io/learning-area/css/web-fonts/web-font-finished.html (1 time) (Note! This may be a new URL 👀)
• https://mdn.github.io/learning-area/css/web-fonts/web-font-start.html (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/MDN/Writing_guidelines/Page_structures/Code_examples
Title: Code examples on MDN

• https://mdn.github.io/learning-area/html/introduction-to-html/getting-started/ (1 time) (Note! This may be a new URL 👀)

[estelle]

Let's revert the example change on learn_web_development/core/accessibility/html/index.md