CSS fallback script checks the wrong element in LinkTagHelper
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- C#
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Description
### Describe the bug
The `` tag generated by LinkTagHelper (as seen [here in the LinkTagHelper class](https://github.com/dotnet/aspnetcore/blob/v5.0.0/src/Mvc/Mvc.TagHelpers/src/LinkTagHelper.cs#L367-L409) and [here in the LinkTagHelper_FallbackJavaScript.js](https://github.com/dotnet/aspnetcore/blob/v5.0.0/src/Mvc/Mvc.TagHelpers/src/js/LinkTagHelper_FallbackJavaScript.js)) references the last script tag on the page, and then takes the `.previousElementSibling` of that HTML element. If I have any other script tags on the page, this fallback script will fail and will always load in the fallback source.
### To Reproduce
Will be using PaperCSS as example here <https://www.getpapercss.com/>, hosted via UNPKG.
1. Create ASP.NET MVC project, ex:
```sh
dotnet new mvc -n LinkTagHelperIssue
```
2. Create directory for fallback file at `wwwroot/lib/papercss/dist`, ex:
```sh
mkdir -pv LinkTagHelperIssue/wwwroot/lib/papercss/dist
```
3. Download <https://unpkg.com/papercss@1.8.1/dist/paper.min.css> and place the downloaded file at `wwwroot/lib/papercss/dist/paper.min.css`, ex:
```sh
wget 'https://unpkg.com/papercss@1.8.1/dist/paper.min.css' -O LinkTagHelperIssue/wwwroot/lib/papercss/dist/paper.min.css
```
4. Edit `Views/Shared/_Layout.cshtml`, add the following to the end of the `<head>` tag:
```cshtml
<link rel="stylesheet"
href="https://unpkg.com/papercss@1.8.1/dist/paper.min.css"
integrity="sha384-R9lNp8nWuBGkBw8HAeT4RYv8Fmj1mD45av7OFm17UfasvI47Yuk/Sy8wUO5p/184"
crossorigin="anonymous"
asp-fallback-href="~/lib/papercss/dist/paper.min.css"
asp-fallback-test-class="paper" asp-fallback-test-property="border" asp-fallback-test-value="1px solid #c1c0bd" />
```
5. Run the project, ex:
```sh
dotnet run -p LinkTagHelperIssue
```
6. Open the page in web browser, for example Chrome. Open the inspector tool and look inside the `<head>` tag, and see that there are two `<link>` tags referencing `paper.min.css`, but one from `unpkg.com` and one from the same site:
### Further technical details
- ASP.NET Core version: 5.0.0 (have NOT tested on any other version though)
- Ran this from dotnet CLI on both Windows and Ubuntu, and from VS on Windows
- VS: Microsoft Visual Studio Community 2019, Version 16.8.2
<details>
<summary>dotnet --info on WSL instance I've tried this on</summary>
```
❯ dotnet --info
.NET SDK (reflecting any global.json):
Version: 5.0.100
Commit: 5044b93829
Runtime Environment:
OS Name: ubuntu
OS Version: 20.10
OS Platform: Linux
RID: ubuntu.20.10-x64
Base Path: /usr/share/dotnet/sdk/5.0.100/
Host (useful for support):
Version: 5.0.0
Commit: cf258a14b7
.NET SDKs installed:
3.1.404 [/usr/share/dotnet/sdk]
5.0.100 [/usr/share/dotnet/sdk]
.NET runtimes installed:
Microsoft.AspNetCore.App 3.1.10 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.10 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
```
</details>
<details>
<summary>dotnet --info from Windows instance I've tried this on</summary>
```
.NET SDK (reflecting any global.json):
Version: 5.0.100
Commit: 5044b93829
Runtime Environment:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.100\
Host (useful for support):
Version: 5.0.0
Commit: cf258a14b7
.NET SDKs installed:
3.0.101 [C:\Program Files\dotnet\sdk]
3.1.100 [C:\Program Files\dotnet\sdk]
3.1.302 [C:\Program Files\dotnet\sdk]
5.0.100 [C:\Program Files\dotnet\sdk]
.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
```
</details>
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