Add fallback tests based on css pseudo-elements

[dasblinkenlight]

Motivation:

CSS for a popular CDN package Bootstrap Icons consists entirely of pseudo-element definitions, such as below:

.bi::before {
  display: inline-block;
  font-family: bootstrap-icons !important;
   ...
}

The current implementation of the ASP.NET fallback tester offers no support for pseudo-elements, because the JavaScript for the tester calls a single-parameter overload of getComputedStyle. To test pseudo-elements, we need to pass the name of the the pseudo-element to be tested as a second parameter of getComputedStyle.

Solution:

We add support for testing pseudo-elements by setting an asp-fallback-test-pseudo-element attribute:

...
asp-fallback-test-class="bi"
asp-fallback-test-pseudo-element="before"
...

We then pass the value of the asp-fallback-test-pseudo-element as the second parameter to the fallback tester function, which then forwards it to getComputedStyle; if no value is specified, we pass null.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[dasblinkenlight]

@mkArtakMSFT @javiercn
This looks like a reasonably isolated task to me, so I would like to try my hand at a pull request. I have a few questions before I start:

• Do you agree that implementing this feature is a reasonable project to start contributing to aspnet.core?
• Would you happen to know which minifier/uglifier was used on the LinkTagHelper_FallbackJavaScript.js from the js folder to arrive at the minified version in the compiler/resources folder? JSCompress produces a nearly identical output, except for the names of the variables.

Could either one of you please let me know?

[lonix1]

For future reference, here is a link to the package, as an example. Every single selector uses a pseudo element.

[lonix1]

@dasblinkenlight Thanks for the tip regarding the getComputedStyle override. Until this is fixed, I've devised a workaround - you can see it on StackOverflow.