H2 — Chapter 8 - Images

Criterion H2 : Descriptive ALT Tag — guide + checklist

PART 1 - Fundamentals Chapter 8 - Images Keyword : balise alt descriptive

This criterion seems “simple,” but it causes many discrepancies in production.

The **H2 — Descriptive ALT Tag** criterion is part of our SEO checklist (335 criteria). Here, you have a **practical** method to check and fix it — with a concrete example.

What exactly this criterion covers

This criterion seems “simple,” but it causes many discrepancies in production.

**H2 — Descriptive ALT Tag** (Chapter 8 - Images): Content AND context description, max 125 characters, keyword if natural, alt='' if decorative

Why it matters (SEO + UX)

Why it matters: it’s a UX point that eventually translates into SEO. When poorly applied, we often observe: ambiguity (wrong associated query), duplication between pages, or loss of impression performance.

On volume-generated sites, this criterion also serves as a **safeguard**: a stable rule prevents 1,000 errors at once.

How to check (step by step)

Approach: validation via Search Console (real data). Recommended tool: **Lighthouse**.

  1. Open the page in Chrome → DevTools → Performance/Network tab.
  2. Run Lighthouse and note the main weak point.
  3. Check if the problem repeats on mass-generated pages.

Tip: first isolate 10 “representative” URLs (top pages + generated pages) before scaling the fix.

How to fix properly

Strategy: make a “clean” fix (no patch), then measure.

  • Convert to WebP/AVIF + compress (Squoosh/ImageOptim).
  • Add width/height to avoid CLS.
  • Write a contextual alt (useful, not keyword-stuffed).

Then: re-crawl 50–200 URLs, then monitor Search Console for 7–14 days (impressions/CTR/indexing).

Concrete example (illustrative)

Example (illustrative):

  • **Context**: service landing page for insurance in Algiers
  • **Before**: PNG images 2–4 MB, no explicit dimensions.
  • **After**: WebP/AVIF conversion + dimensions + descriptive alt (context: service landing page).
  • **Note**: Goal: reduce weight and improve rendering (and sometimes image search).

Checklist to tick

  • [ ] Optimized weight
  • [ ] Respects: alt='' if decorative
  • [ ] Modern format
  • [ ] Defined dimensions
  • [ ] Useful and contextual alt
FAQ

Frequently asked questions — H2

What is the most frequent error on “Descriptive ALT Tag”?

Fixing an isolated page without correcting the template/import: the error returns at the next generation.

Which tool is fastest for large-scale checking?

For this type of criterion, a crawl (e.g., Screaming Frog) + targeted verification in Lighthouse is generally the fastest combo.

How to prevent this from recurring on 10K generated pages?

Freeze an auto-generation rule (title/structure/schema/URLs) + add automatic control (crawl or test) before production import.

Ready to go from theory to action?

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