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Course 279 · Digital course 42

Measure and Improve Web Page Performance

Build a before/after optimization record for a supplied local page, with resource-size accounting, reserved image dimensions, fewer blocking resources and repeatable browser measurements. Work from supplied original examples and check changed cases.

Before you start

Review Build your first website and introductory JavaScript.

Build Your First Website See everything you’ll need

Free download · Original practice data · No external account required. Download the starter pack

What you’ll leave with

A before/after optimization record for a supplied local page, with resource-size accounting, reserved image dimensions, fewer blocking resources and repeatable browser measurements.

Start course Start without an account. Sign in to save lesson progress across devices.

Your project

Start with the practice files. Build your project one lesson at a time.

Practice files are free without an account. Sign in to save your project record in My Proof: a description, checklist, and optional work link. Keep the original files in your own app or notebook; saving a project record does not upload them.

  1. Get your materialsDownload practice filesSupplied practice cases, a blank working record, complete lesson instructions, and annotated worked examples.
  2. Make your projectView the project checklist Work in your own notebook, document, or software. Use the supplied practice examples when you prefer.
  3. Keep a private recordSave a project record Sign in to save a description and checklist, then return to update them.
Course handbook

Know the route before you begin.

Here’s what you’ll need and what you’ll make. You can return to this guide during any lesson.

Who this is for

Learners ages 14+ and adults ready for the named prerequisite; no employment or private records required.

Prepare before lesson one
  • Review Build your first website and introductory JavaScript.
  • Use a current browser with developer tools and Python 3 for local serving and supplied file assertions. No external packages, paid tools, accounts, or network services required.
  • Keep both original bundles in a fresh folder and record measurement conditions.
Your final project

Finish with proof you can use.

A before/after optimization record for a supplied local page, with resource-size accounting, reserved image dimensions, fewer blocking resources and repeatable browser measurements.

Save a project record in My Proof
See project checklist and review criteria

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • Deliver an inventory and measurement table separating bytes, requests, timing, and content checks.
  • Produce a delivered-image byte comparison and a preserved-content visual check.
  • Verify aspect-ratio reservation and responsive fit, with an appropriate vector/raster source explanation.
  • Produce a before/after CSS request-count and preserved-style comparison.
  • Retain five actual samples, a correctly calculated median, and an explicit timed-block boundary.
  • Deliver a measured, preserve-content before/after packet with performed and unavailable checks separated.

Review and revise

Use these criteria to check your work and choose what to improve.

  • The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
  • Normal, boundary and rejected cases are recorded with expected versus observed results.
  • The changed case has a correct explanation and does not rely on hidden state.
  • The project includes source, outputs, instructions and honest tool, privacy and verification limits.
Three ideas to help you along the way
Three useful ideas

The rules behind the steps.

01

State the contract

Name inputs, permitted states and observable outputs before editing the implementation.

02

Preserve the evidence

Keep the actual source, changed input and expected/observed record so someone can repeat the work.

03

Test the boundary

Check a rejection or changed case and distinguish local practice from real service or native-tool behavior.

What do these words mean?
Key words

Key terms worth knowing.

payload
The data carried by a selected resource.
aspect ratio
The relationship between image width and height.
blocking resource
A resource that can delay a rendering step.
median
The center value of an ordered sample set.
confounding
Another changed condition that prevents a clear causal comparison.
Your route

6 lessons.
One useful project.

Work in order the first time. Each lesson makes something the next lesson can use.

  1. 1
    Up next

    Inventory resources before guessing speed

    Deliver an inventory and measurement table separating bytes, requests, timing, and content checks.

  2. 2
    29 min

    Remove unused payload while preserving content

    Produce a delivered-image byte comparison and a preserved-content visual check.

  3. 3
    29 min

    Reserve image space and choose image sources

    Verify aspect-ratio reservation and responsive fit, with an appropriate vector/raster source explanation.

  4. 4
    29 min

    Reduce blocking resources without changing styles

    Produce a before/after CSS request-count and preserved-style comparison.

  5. 5
    29 min

    Measure repeated timings with honest units

    Retain five actual samples, a correctly calculated median, and an explicit timed-block boundary.

  6. 6
    29 min

    Deliver a reproducible optimization decision

    Deliver a measured, preserve-content before/after packet with performed and unavailable checks separated.

Use the tools

Tools to help you practice.

See all tools
ZIP · practice files

Measure and Improve Web Page Performance practice files

Supplied practice cases, a blank working record, complete lesson instructions, and annotated worked examples.

Text worksheet

Your course project worksheet

A downloadable text worksheet with each lesson’s working output, space for your answers, and the shared final-project checklist. Save your own copy.

Source transparency

Sources and further study.

Explore the references behind the lessons and take your learning further. Source notes explain access and reuse.

developer.mozilla.org — Performance
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

developer.mozilla.org — Responsive_images
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

developer.mozilla.org — now
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

Ready when you are

Start with the first useful move.

You’ll always know what to do, what to make, and where to go next.

Start lesson one