Optimization

Script Loading

Aspect	inline script	sync	async	defer	external + type="module"
Visualization
Present in Tag	<script>...</script>	<script src="..."></script>	<script async src="..."></script>	<script defer src="..."></script>	<script type="module" src="..."></script>
Loading Behavior	script is executed immediately when encountered	script is fetched immediately as the browser encounters the tag	script is fetched in parallel with HTML parsing	script is fetched in parallel with HTML parsing	fetched in parallel
Execution Timing	immediate execution	script is executed immediately after it's downloaded, before continuing parsing the HTML document	script is executed as soon as it's downloaded, even if the HTML parsing is not finished	script is executed in order after HTML parsing is fully complete, just before DOMContentLoaded	deferred by default, executed after parsing
Execution Order	based on document location	preserved, as per DOM order	not preserved - random order	preserved	preserved (per top-level await and import graph)
Blocking Behavior	blocks HTML parsing	blocks HTML parsing	may block parsing at execution	does not block parsing	deferred behavior
Notes	avoid for large code blocks; inline code can't use async or defer	default behavior; loading blocks rendering	non-deterministic execution order; may lead to race conditions	most preferred for non-blocking DOM-safe execution	always deferred by spec; uses strict mode automatically
Use Cases	critical inline JS; not affected by async / defer	scripts that must run immediately, and manipulate DOM elements above them	analytics, tracking, or other scripts that don't depend on DOM or other scripts	scripts that depend on DOM elements, and must maintain execution order	modern ES Modules and top-level imports

Optimization Strategies

Core

Strategy	Description
Minimize HTTP Requests	Fewer files mean faster load times due to reduced browser requests
Reduce File Sizes	Smaller assets (images, code, videos) download more quickly
Optimize Critical Rendering Path (CRP)	Improve how fast key content appears and becomes usable
Leverage Caching	Store files in-browser or via CDN to speed up repeat visits
Prioritize User Experience	Make the site feel fast and responsive, even if not fully loaded
Reduce Server Response Time (SSR)	Faster backend response improves overall frontend speed

JS

Strategy	Description
Minification	Minify code by removing unnecessary characters and shortening variable/function names
Compression	Serve JS files with Gzip or Brotli compression enabled
Code Splitting/Bundling	Bundle multiple JS files into a single file to reduce HTTP requests, and use code splitting to load smaller chunks on demand
Lazy Loading (Dynamic Imports)	Load JS modules or components on demand, such as using import('./my-component.js') to load the component only when the promise resolves
Tree Shaking (Dead Code Elimination)	Eliminate unused code from the final JS bundle to enable effective tree shaking
Deferring Non-Critical JavaScript	Use async or defer attributes on <script> tags for non-essential JavaScript
Optimize JavaScript Execution	Avoid long-running tasks on the main thread to maintain browser responsiveness by breaking up large tasks with setTimeout or requestAnimationFrame, using Web Workers for heavy computations, and applying debouncing or throttling for frequently fired events like scroll or resize
Efficient DOM Manipulation	Minimize direct DOM manipulation, batch updates with documentFragment, use requestAnimationFrame for visual updates, and avoid layout thrashing by batching read/write operations to prevent forced synchronous reflows
Remove Unnecessary Libraries/Dependencies	Audit your project for unused or bloated third-party libraries. Consider using lighter alternatives or writing custom code if feasible

CSS

Strategy	Description
Minification	Remove unnecessary characters like whitespace, comments, and line breaks from CSS files
Compression	Serve CSS files with Gzip or Brotli compression enabled on the server. This reduces the file size transferred over the network
Critical CSS (Above-the-Fold CSS)	Extract the minimum CSS for "above-the-fold" content, inline it in the HTML <head> for faster rendering, and load the remaining CSS asynchronously
Remove Unused CSS	Analyze your project to identify and remove CSS rules that are not used on any page. This drastically reduces file size
Optimize Selectors and Rules	Use simpler selectors for faster matching, avoid unnecessary qualifications of IDs or classes, and limit @import rules to prevent blocking parallel downloads, favoring link tags instead
Use CSS Sprites	Combine small background images into a single file and use CSS background-position to display segments, reducing the number of HTTP requests

HTML

Srategy	Description
Remove Comments & Whitespace	Reduces file size by eliminating non-essential content
Eliminate Redundant Tags	Keep HTML semantic and free of unnecessary elements
Avoid Large Inline CSS/JS	Prevents bloated HTML and allows for better caching
Validate HTML	Ensures consistent rendering and fewer browser issues
Place CSS in head	Enables faster rendering and avoids unstyled content flashes
Place JS at Bottom or Use async	async loads and runs scripts as soon as ready; execution order not guaranteed
Use defer for Dependent Scripts	Loads scripts in order after HTML parsing, before DOMContentLoaded

Media

Srategy	Description
Image Compression	Compress images to reduce file size without significant quality loss
Choose the Right Image Format	Use JPEG for photographs, PNG for transparency or sharp-edged images like logos, SVG for resolution-independent vector graphics, and modern formats like WebP or AVIF for superior compression, with fallbacks for older browsers
Responsive Images	Serve image sizes based on device, resolution, and viewport using srcset and sizes in <img> for automatic selection, and utilize the <picture> element for complex layouts or art direction
Lazy Loading Images and Videos	Load media as it nears the viewport by using loading="lazy" on <img> and employ the Intersection Observer API for better control or support in older browsers
Image Dimensions	Always specify width and height on <img> and <video> to prevent layout shifts and reserve space during loading
Optimize Video Delivery	Compress videos and use modern formats like WebM and MP4 (H.264/H.265), stream instead of forcing full downloads, and preload metadata or small segments for faster playback start

Font

Strategy	Description
Reduce Font File Size	Subset fonts to include only necessary characters, use WOFF2 for modern, compressed formats with fallbacks like WOFF and TTF for older browsers, and load only required font weights and styles to remove unused styles
Font Loading Strategy	Use the font-display property to control font loading and prevent "Flash of Invisible Text" FOIT, "Flash of Unstyled Text" FOUT, and "Flash of Faux Text" FOFT; additionally, preload critical fonts using <link rel="preload" as="font" crossorigin href="myfont.woff2"> in the <head> to fetch important fonts early

Server-Side

Strategy	Description
Content Delivery Network (CDN)	Serve static assets like images, CSS, and JavaScript via a CDN to reduce latency by delivering content from the closest globally distributed server to the user
Browser Caching (HTTP Caching Headers)	Configure headers like Cache-Control, Expires, ETag, and Last-Modified to instruct browsers on how long to cache static resources, reducing repeated downloads for returning visitors
HTTP/2 or HTTP/3	Ensure your server supports HTTP/2 or HTTP/3 for improved performance over HTTP/1.1, featuring multiplexing (multiple requests/responses over one connection), header compression (reducing request overhead), and server push (proactively sending likely-needed resources, though more limited in HTTP/3)
Prefetching and Preloading (Resource Hints)	Use rel attributes in <link> tags to enhance perceived performance by utilizing preload to fetch critical resources early (e.g., fonts), prefetch for resources likely needed for future navigation, preconnect to open early connections to important third-party origins, and dns-prefetch to resolve DNS early for external domains
Reduce Redirects	Minimize unnecessary redirects (e.g., HTTP to HTTPS, www to non-www) as each redirect adds an extra round-trip delay, slowing down page loading
Server-Side Rendering (SSR) / Static Site Generation (SSG)	Improve Time To Interactive (TTI) by delivering HTML directly through Server-Side Rendering (SSR), where HTML is rendered on the server and sent fully formed to the browser, or Static Site Generation (SSG), where HTML files are generated at build time and served statically (e.g., via a CDN) for extremely fast load times

Advanced

Strategy	Description
WebAssembly (Wasm)	For highly computational tasks like video editing and gaming, WebAssembly enables near-native performance by allowing code compiled from languages such as C++, Rust, and Go to run in the browser
Service Workers	Enable offline support by caching assets and API responses to serve cached content for fast repeat visits, and implement complex caching strategies like cache-first or network-first
Performance Budgeting	Define limits on metrics such as JavaScript size, First Contentful Paint, and Time To Interactive (TTI), and enforce budgets through CI/CD to prevent performance regressions
Continuous Performance Monitoring (RUM & Synthetic)	RUM (Real User Monitoring) captures real user performance data such as Core Web Vitals and load times, while Synthetic Monitoring simulates user sessions in test environments using tools like Lighthouse and WebPageTest in CI
User-Centric Performance Metrics (Core Web Vitals)	Optimize for metrics that impact real user experience, including LCP (Largest Contentful Paint) for perceived load speed, CLS (Cumulative Layout Shift) for visual stability, and INP (Interaction to Next Paint) as a new responsiveness metric measuring full input-to-render delay
Third-Party Script Optimization	Third-party scripts (e.g., analytics, chat, ads) can harm performance; therefore, audit to remove unnecessary scripts, use async or defer attributes, lazy load scripts only when needed (e.g., show chat on click), and host common libraries locally for better control