Use callout without custom Jekyll plugin.

_includes/callout-danger-async-methods.md

@@ -1,7 +1,8 @@
-{% callout danger %}
+{% capture callout %}
 #### Asynchronous methods and transitions
 
 All API methods are **asynchronous** and start a **transition**. They return to the caller as soon as the transition is started but **before it ends**. In addition, a method call on a **transitioning component will be ignored**.
 
 [See our JavaScript documentation for more information.]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/javascript/)
-{% endcallout %}
+{% endcapture %}
+{% include callout.html content=callout type="danger" %}

_includes/callout-info-mediaqueries-breakpoints.md

@@ -1,3 +1,4 @@
-{% callout info %}
+{% capture callout %}
 Note that since browsers do not currently support [range context queries](https://www.w3.org/TR/mediaqueries-4/#range-context), we work around the limitations of [`min-` and `max-` prefixes](https://www.w3.org/TR/mediaqueries-4/#mq-min-max) and viewports with fractional widths (which can occur under certain conditions on high-dpi devices, for instance) by using values with higher precision for these comparisons.
-{% endcallout %}
+{% endcapture %}
+{% include callout.html content=callout type="info" %}

_includes/callout-warning-color-assistive-technologies.md

@@ -1,5 +1,6 @@
-{% callout warning %}
+{% capture callout %}
 ##### Conveying meaning to assistive technologies
 
 Using color to add meaning only provides a visual indication, which will not be conveyed to users of assistive technologies – such as screen readers. Ensure that information denoted by the color is either obvious from the content itself (e.g. the visible text), or is included through alternative means, such as additional text hidden with the `.sr-only` class.
-{% endcallout %}
+{% endcapture %}
+{% include callout.html content=callout type="warning" %}

_includes/callout.html

@@ -0,0 +1,5 @@
+{% comment %} allowed: info danger warning {% endcomment %}
+{% assign css_class = include.type | default: "info" %}
+<div class="bd-callout bd-callout-{{ css_class }}">
+  {{ include.content | markdownify }}
+</div>