Minor Blazor form validation improvements

[oroztocil]

A few small additions and phrasing improvements I realized I want to do after reading through the docs on the live website.

Summary

• Documents that replacing EditForm.Model resets modification and validation state.
• Explains how ValidationMessage.For identifies fields through FieldIdentifier.
• Shows how to apply validation classes to surrounding markup with EditForm.Context and FieldCssClass.
• Adds a property-level AsyncValidationAttribute example with dependency injection, cancellation, member association, and synchronous fallback.

Verification

• The new .NET 11 async validation example builds with zero warnings and errors.

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Internal previews

File	Preview link
aspnetcore/blazor/forms/binding.md	Learn preview
aspnetcore/blazor/forms/validation.md	Learn preview
aspnetcore/fundamentals/validation.md	Learn preview

Build report

[Copilot]

Copilot review overview

🟡 Changes recommended

The fundamentals article needs its metadata date updated, and one spacing error remains.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 2

Open (2)

• Remove extra space before Context · New
• Update ms.date to 09/23/2026 · New

What changed in this PR

Improves Blazor form validation guidance and adds a .NET 11 asynchronous validation example.

Changes:

• Documents EditContext lifecycle and field identity.
• Demonstrates CSS classes on surrounding markup.
• Adds property-level asynchronous validation.

File	Description
aspnetcore/​blazor/​forms/​binding.md	Explains model replacement behavior.
aspnetcore/​blazor/​forms/​validation.md	Clarifies field identity and CSS styling.
aspnetcore/​fundamentals/​validation.md	Adds an asynchronous validation attribute example.

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[oroztocil]

@copilot Fix the code for all comments in this review thread.

When a review comment includes a suggested change, apply the suggestion exactly.

Do not make changes beyond what is described in the linked review thread.

[Copilot]

@copilot Fix the code for all comments in this review thread.

When a review comment includes a suggested change, a...

Fixed in 8a1d922: removed the extra space before Context and updated ms.date to 09/23/2026 in fundamentals/validation.md.

[guardrex]

One tiny XREF change spotted.

[guardrex]

BTW ... I'm trying to get my XREF gen tool fixed. Can you provide an approval on ...

dotnet/blazor-samples#671

... and then it will start working properly again ...

https://dotnet.github.io/blazor-samples/

[guardrex]