Prevent tree-shaking CSS selectors for dynamic attributes

[westonruter]

This fixes an issue reported on the support forum:

I’m only trying to style an expanded menu inside of a sidebar. I’m using standard AMP elements, including amp-accordion. The inline style that gets added by the plugin is automatically removing the styles that would allow this, for example:

section.menu-section[expanded] {
    background: green;
}

The plugin isn’t stripping out any other content.

In #1959 (and #2017, #2642, #2710) tree shaking for CSS selectors was made aware of the class names for AMP components which can be dynamically added/removed from a page. Even though the selector with such a class name may not exist at the initial time of rendering the page, the selector may very well apply later. The same logic needed to be extended to attributes which are added/removed to AMP components. In particular:

• amp-sidebar: The open attribute is dynamically added when the sidebar is opened.
• amp-accordion: The expanded attribute is added when a section is opened by the user.
• amp-live-list: The data-tombstone attribute can appear in an amp-live-list's contents during the life of a page.