Add mixed-value state styling to checkboxes

In this PR

Added :indeterminate pseudo-class selector for input[type="checkbox"], including a disabled state.
Added checkbox-mixed-value.svg and checkbox-mixed-value-disabled.svg[^1]

💭 Reasoning

This state has been added to add additional styling present on pre-2000s Windows OSes, and to prevent browser default styling from occurring when a checkbox is set to indeterminate.

From The Windows Interface Guidelines — A Guide for Designing Software, Chapter 7 ("Menus, Controls, and Toolbars"), page 143:

If you use a check box to display the value for the property of a multiple selection whose values for that property differ (for example, for a text selection that is partly bold), display the check box in its mixed-value appearance, a checkerboard pattern inside the box, as shown in Figure 7.12.

This further allows users of 98.css to style their HTML markup and/or applications to match Windows 98.

[^1]: Filenames chosen from Windows Interface Guidelines

jdan / 98.css

Add mixed-value state styling to checkboxes #196

In this PR

💭 Reasoning