Post-Render Transformers — HTML Processing Pipeline

The transformer pipeline lets you modify rendered HTML before it reaches consumers (email channels, PDF generators). This is particularly useful for email templates, where you need table-based layout, inline CSS, and Outlook conditional comments — concerns that template authors should not deal with manually.

IRenderedContentTransformer interface

public interface IRenderedContentTransformer
{
    int Order { get; }
    bool CanTransform(DocumentFormat format);
    Task<string> TransformAsync(string content, DocumentFormat format, CancellationToken ct);
}

Transformers run as an ordered mutation chain: each receives the output of the previous one and returns modified content. Use CanTransform to restrict a transformer to specific formats (e.g., HTML-only, skip PDF/Excel).

Registering a custom transformer

services.AddRenderedContentTransformer<HtmlMinificationTransformer>();

Writing a custom transformer

public sealed class HtmlMinificationTransformer : IRenderedContentTransformer
{
    public int Order => 300;
    public bool CanTransform(DocumentFormat format) => format == DocumentFormat.Html;

    public Task<string> TransformAsync(
        string content, DocumentFormat format, CancellationToken ct)
    {
        // Your minification logic here
        return Task.FromResult(Minify(content));
    }
}

Ordering conventions

Range	Purpose	Example
100	Structural transformation	MJML compilation
200	CSS processing	Inlining, purging
300+	Cosmetic	Minification, cleanup

The built-in MjmlTransformer (Order=100) runs first to convert MJML markup into table-based HTML. Application-level transformers should use Order 200+ to process the already-compiled HTML. See MJML Email Templates for details on the MJML transformer, and Rendering Pipeline for how transformers fit into the five-stage pipeline.

See also

• Overview — package structure and setup
• Rendering Pipeline — the five-stage pipeline
• MJML Email Templates — the built-in Order=100 transformer
• Scriban Engine — what runs before transformers
• Configuration & API Reference — ITextTemplateTransformer contract