This is an impactful enough change that it needs its own docs information in preview 9. This is a summary for now, and I'll write detailed guidance later.

Summary

In 3.0-preview9 and later, the set of bindings and event handlers available for HTML elements are defined in the Microsoft.AspNetCore.Components.Web assembly and based on the Microsoft.AspNetCore.Components.Web namespace.

Migration Guidance

Blazor WebAssembly and Libraries

When migrating an application to preview 9 or later, make sure to add a reference to the Microsoft.AspNetCore.Components.Web nuget package. Next, open the application's root _Imports.razor and add @using Microsoft.AspNetCore.Components.Web.

Blazor Server-Side

When migrating an application to preview 9 or later, open the application's root _Imports.razor and add @using Microsoft.AspNetCore.Components.Web.

note: Server-Side Blazor applications get a reference to Microsoft.AspNetCore.Components.Web implicitly without any additional package references.

Troubleshooting guidance

With the correct references and @using, event handlers like @onclick and @bind should be bold font, and colorized as shown below when using Visual Studio.

If @bind or @onclick is colorized as a normal HTML attribute, like the below, then the @using is missing.

You may also see errors like:

CS0169	The field 'Index.text' is never used
CS0428	Cannot convert method group 'Submit' to non-delegate type 'object'. Did you intend to invoke the method?

Try adding @using Microsoft.AspNetCore.Components.Web to the file. If you now see an error like the following, this means that the reference to the Microsoft.AspNetCore.Components.Web package is missing from the project.

CS0234	The type or namespace name 'Web' does not exist in the namespace 'Microsoft.AspNetCore.Components' (are you missing an assembly reference?)

If adding the using fixed the problem, consider moving to the application's root _Imports.razor so it will apply to all files.