The asp-fallback-href attribute gives a link tag helper a local stylesheet URL to use when a CDN-hosted stylesheet is unavailable. It works with href and fallback-test attributes that let browser-side code verify whether the CDN stylesheet loaded. section 26.4.2
Bootstrap CDN with a local fallback
In a shared layout, set href to the Bootstrap CDN URL and asp-fallback-href to /lib/bootstrap/css/bootstrap.min.css. Add asp-fallback-test-class="btn", asp-fallback-test-property="display", and asp-fallback-test-value="inline-block". The tag helper emits the CDN link, a test meta element, and JavaScript that checks the CSS property before writing the local fallback link. section 26.4.2
Table 26.6. The built-in tag helper attributes for link elements (view table figure)
| Name |
Description |
|---|---|
| asp-href-include |
This attribute is used to select files for the href attribute of the output element. |
| asp-href-exclude |
This attribute is used to exclude files from the href attribute of the output element. |
| asp-append-version |
This attribute is used to enable cache busting, as described in the “Understanding Cache Busting” sidebar. |
| asp-fallback-href |
This attribute is used to specify a fallback file if there is a problem with a CDN. |
| asp-fallback-href-include |
This attribute is used to select files that will be used if there is a CDN problem. |
| asp-fallback-href-exclude |
This attribute is used to exclude files from the set that will be used when there is a CDN problem. |
| asp-fallback-href-test-class |
This attribute is used to specify the CSS class that will be used to test the CDN. |
| asp-fallback-href-test-property |
This attribute is used to specify the CSS property that will be used to test the CDN. |
| asp-fallback-href-test-value |
This attribute is used to specify the CSS value that will be used to test the CDN. |
Use the link-element attribute table to compare asp-fallback-href with fallback file selection and fallback test attributes. section 26.4.2
