Priorização de conteúdo para obter velocidade
Implemente técnicas como `async`, `defer` e dicas de recursos para priorizar conteúdo crítico e melhorar o desempenho percebido.
Priorização de conteúdo para obter velocidade é uma aula grátis de Web Performance Optimization & Lighthouse no CoddyKit. Esta é a aula 3 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Web Performance Optimization & Lighthouse, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Web Performance Optimization & Lighthouse inclui 4 aulas no total.
Partes desta aula ainda não foram traduzidas e aparecem em inglês.
Why Prioritize Content?
When a browser loads a webpage, it follows a sequence of steps to render content. This is often called the Critical Rendering Path (CRP).
Optimizing the CRP means delivering the most important content to the user as quickly as possible. This improves perceived performance – how fast a user *feels* the page is loading.
Blocking Scripts Slow Down
By default, when the browser encounters a <script> tag, it pauses parsing the HTML, fetches the script, executes it, and then resumes parsing. This is called a render-blocking script.
This can significantly delay how quickly users see content, especially if the script is large or on a slow network.
<html>
<head>
<title>Blocking Script</title>
<script>
// This script blocks HTML parsing
console.log("Blocking script started.");
// Simulate a long task
let start = performance.now();
while (performance.now() - start < 700) { /* busy wait */ }
console.log("Blocking script finished.");
</script>
</head>
<body>
<h1>Page Content</h1>
<p>This content appears <b>after</b> the script finishes.</p>
</body>
</html>Introducing `defer`
The defer attribute tells the browser to download the script in the background without blocking HTML parsing. Once the HTML is fully parsed, the deferred scripts execute in the order they appear in the document.
- Non-blocking: HTML parsing continues.
- Order preserved: Scripts execute in source order.
- Execution time: After HTML parsing, before
DOMContentLoaded.
`defer` in Action
Using defer is ideal for scripts that depend on the DOM being fully available, like analytics or UI initialization logic.
See how the 'Page Content' appears before the deferred script executes in the console.
<html>
<head>
<title>Deferred Script Example</title>
<script defer>
console.log("Deferred script 1 started.");
</script>
<script defer>
// This script will run after script 1
console.log("Deferred script 2 started.");
</script>
</head>
<body>
<h1>Page Content</h1>
<p>This content appears immediately while scripts are downloading.</p>
<script>
console.log("HTML parsing complete.");
</script>
</body>
</html>Introducing `async`
The async attribute also tells the browser to download the script in the background. However, async scripts execute as soon as they are downloaded, potentially before HTML parsing is complete.
- Non-blocking: HTML parsing continues.
- Order NOT preserved: Scripts execute as soon as they're ready.
- Execution time: Asynchronously, as soon as possible.
`async` vs. `defer`
Choosing between async and defer depends on your script's needs:
- Use
asyncfor independent scripts (e.g., third-party ads, tracking) where execution order doesn't matter. - Use
deferfor scripts that need the DOM or depend on other deferred scripts, ensuring order is maintained.
<html>
<head>
<title>Async vs Defer</title>
<script async>
console.log("Async script executed!");
</script>
<script defer>
console.log("Defer script executed!");
</script>
<script>
console.log("HTML parsing continues, then inline script runs.");
</script>
</head>
<body>
<h1>Async & Defer Demo</h1>
<p>Check the console to see the execution order.</p>
</body>
</html>Resource Hint: `preconnect`
Resource hints give the browser clues about resources it will need soon, allowing it to start fetching them earlier.
<link rel="preconnect"> tells the browser to establish a connection to another domain early. This includes DNS lookup, TCP handshake, and TLS negotiation, saving precious milliseconds for critical third-party resources.
Resource Hint: `preload`
<link rel="preload"> tells the browser to fetch a resource that is critically important for the current navigation as soon as possible. This is useful for resources like fonts, critical CSS, or images that are discovered late by the parser.
<html>
<head>
<title>Preload Example</title>
<link rel="preload" href="data:text/css;base64,Ym9keSB7IGJhY2tncm91bmQtY29sb3I6IGxpZ2h0Ymx1ZTsgfQ==" as="style">
<style>
/* This is a simple inline style */
body { margin: 20px; }
</style>
<style>
/* The preloaded style will apply */
@import url("data:text/css;base64,Ym9keSB7IGJhY2tncm91bmQtY29sb3I6IGxpZ2h0Ymx1ZTsgfQ==");
</style>
</head>
<body>
<h1>Preloaded Style</h1>
<p>The CSS for the light blue background is preloaded, ensuring it's available early.</p>
</body>
</html>Resource Hint: `prefetch`
<link rel="prefetch"> tells the browser to fetch a resource that might be needed for a future navigation. It's a lower priority hint than preload and is great for speeding up subsequent page loads.
For example, if you know a user is likely to click a specific link, you can prefetch the resources for that page.
Prioritization Quiz
Which of the following statements about script loading and resource hints are TRUE?
Recap: Faster Page Loads
We've learned powerful techniques to prioritize content and speed up your web pages:
- `defer` for non-blocking script loading, preserving execution order.
- `async` for non-blocking, independent script execution.
- `preconnect` to warm up connections to other domains.
- `preload` to fetch critical resources early for the current page.
- `prefetch` to fetch resources for future navigations.
By strategically using these, you can significantly improve your website's perceived performance and user experience!
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O que vou aprender em “Priorização de conteúdo para obter velocidade”?
Implemente técnicas como `async`, `defer` e dicas de recursos para priorizar conteúdo crítico e melhorar o desempenho percebido. Você pratica Web Performance Optimization & Lighthouse com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.
Preciso ter experiência prévia para começar Web Performance Optimization & Lighthouse?
Nenhuma experiência prévia é necessária. Web Performance Optimization & Lighthouse no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 3 de 4.
Quanto tempo leva a aula “Priorização de conteúdo para obter velocidade”?
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Posso escrever e executar código nesta aula de Web Performance Optimization & Lighthouse?
Sim. Cada aula de Web Performance Optimization & Lighthouse inclui um editor de código integrado, então você escreve e executa código real direto no navegador e recebe feedback de IA instantaneamente — nenhuma configuração local necessária.
Todas as aulas deste curso
- Compreensão do caminho crítico
- Recursos que bloqueiam a renderização
- Priorização de conteúdo para obter velocidade
- Pré-Carregamento e Dicas de Recursos