Advanced HTML and CSS transitions: add additional article

advanced_html_css/animation/transitions.md

@@ -88,7 +88,7 @@ What's important is that you have a solid understanding of these concepts and ca
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 1. Read the MDN article for [using CSS transitions](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions), including the links in the [Defining transitions](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions#defining_transitions) section of the article. Make sure you code along with any examples to become familiar with the syntax.
-1. Read the MDN article about [the stacking context](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context) for a deeper understanding.
+1. Read this article about [stacking context](https://www.joshwcomeau.com/css/stacking-contexts/).
 1. Learn some handy hints on [how to create performant CSS animations](https://web.dev/animations-guide/) and check out the link it provides to [CSS Triggers](https://web.archive.org/web/20220727225220/https://csstriggers.com/) to see how an animatable CSS property may affect others. Look at the difference between the `background-color` and `transform` properties, for example.
 1. Read this [interactive guide to transitions](https://www.joshwcomeau.com/animation/css-transitions/).
 1. Learn how to [catch and debug repaint issues](https://dzhavat.github.io/2021/02/18/debugging-layout-repaint-issues-triggered-by-css-transition.html).
@@ -108,6 +108,7 @@ The following questions are an opportunity to reflect on key topics in this less
 
 This section contains helpful links to related content. It isn't required, so consider it supplemental.
 
+- For a deeper dive, check out this [MDN article on the stacking context](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context).
 - Up until now, you have only controlled the speed of your transitions with the help of keyword values like `ease` and `ease-in`. However, CSS allows you to define your own timing functions with the help of timing functions like `steps` and `cubic-bezier`. In specific, [the `cubic-bezier` function](https://developer.mozilla.org/en-US/docs/Web/CSS/easing-function#cubic_b%C3%A9zier_easing_function) is widely used.
 - While the [`cubic-bezier`](https://blog.maximeheckel.com/posts/cubic-bezier-from-math-to-motion/) function has a ton of mathematics, you do not need to delve deep into it apart from a creating a basic idea about its purpose.
 - You can play around with the [CSS Cubic Bezier Generator](https://www.cssportal.com/css-cubic-bezier-generator/) to not only view how the `cubic-bezier` value works in an editable demo, but also to compare the different `transition-timing-function` values side by side.