How do ASP.NET Core's “asp-fallback-*” CDN tag helpers work?

Question

I understand what the asp-fallback-* tag helpers do. What I don\'t understand is how.

For example:

Answer 1

TL;DR:

• A tag is added to the DOM that has a CSS class of sr-only.
• Additional JavaScript is written to the DOM, which:
  1. Locates said element.
  2. Checks whether said element has a CSS property position that is set to absolute.
  3. If no such property value is set, an additional element is written to the DOM with a href of ~/lib/bootstrap/dist/css/bootstrap.min.css.

---

The LinkTagHelper class that runs against your elements inserts a element in the output HTML that is given a CSS class of sr-only. The element ends up looking like this:

The code that generates the element looks like this (source):

builder
    .AppendHtml("");

Unsurprisingly, the value for FallbackTestClass is obtained from the 's asp-fallback-test-class attribute.

Right after this element is inserted, a corresponding