Plugin system for custom analyzers

[Ouranos27]

Problem

The engine ships 8 built-in analyzers (CSS compatibility, spam, accessibility, links, images, inbox preview, size, templates). But teams often have domain-specific rules:

• Brand compliance (logo size, brand colors, approved fonts)
• Internal link policies (no shortened URLs, require UTM params)
• Regulatory requirements (GDPR footer, CAN-SPAM physical address)
• Company-specific Outlook workarounds

Currently there's no way to extend auditEmail() with custom checks.

Proposal

Add a plugin/analyzer API that lets users register custom analyzers:

import { auditEmail, defineAnalyzer } from "@emailens/engine";

const brandCheck = defineAnalyzer({
  id: "brand-compliance",
  name: "Brand Compliance",
  analyze(html, $) {
    const warnings = [];
    // Check logo dimensions
    $("img[src*='logo']").each((_, el) => {
      const width = $(el).attr("width");
      if (!width || parseInt(width) < 200) {
        warnings.push({ severity: "error", message: "Logo must be at least 200px wide" });
      }
    });
    return { score: 100 - warnings.length * 10, warnings };
  },
});

const report = auditEmail(html, { plugins: [brandCheck] });
// report.plugins["brand-compliance"].score → 90

Design Considerations

• Plugins receive the parsed Cheerio $ instance (already available internally)
• Plugin results should follow the same { score, warnings } shape as built-in analyzers
• Should support async analyzers (e.g., fetching remote brand config)
• Plugin order shouldn't matter (analyzers are independent)

Context

On the roadmap. This would make the engine useful for teams with specific email standards beyond generic best practices.