Lexique Web Performance

A/B testing tools allow you to implement changes without modifying the source code and compare multiple versions of the same page on users browsing your site. By measuring the conversion rates corresponding to each version, you can determine if your test is conclusive before making the change in your site's code.

Meaning "Asynchronous JavaScript and XML", this technology allows the browser to communicate with a server asynchronously, avoiding a page reload for the user. The term AJAX is generally associated with the jQuery library but is also used during XHR requests.

In contrast to the front-end, the back-end is the code invisible to the user. It is the submerged part of the iceberg, managing database transactions and the logic behind each user action. The back-end code is executed by the web server.

During high demand on the user's device resources. This is the time during which the web page freezes or responds very slowly to user interactions.

Compression algorithm that reduces the size of textual resources (HTML, CSS, JS, ...). Introduced after Gzip, it offers better compression performance and reduces the download time of compressed resources by the browser.

The Cumulative Layout Shift is a performance metric that measures the visual stability of a web page over time. As one of the Core Web Vitals, CLS is taken into account in Google's CrUX.

Consent Management Platforms are more commonly referred to as cookie banners. The consent management platform allows for the collection of visitors' preferences during their first visit to the site regarding tracking and cookie placement. To comply with GDPR, it must be present on any site that processes personal data. These scripts distort web performance tests, making it essential to find a way to bypass cookie consent banners.

The Chrome User Experience Report is a tool from Google that allows you to determine the performance level of a page or site using real user data. To obtain figures, the compatible browser (Chrome Desktop and Android, to summarize) collects web performance indicators (including Core Web Vitals). The collected data is compiled monthly into a public BigQuery and can be accessed in the Search Console.

The Domain Name System protocol is used by browsers to associate each domain name with the server it is linked to via an addressing system. To associate a domain name with an IP address, the domain owner fills out a DNS zone composed of fields of different types, allowing for email management, pointing to web servers, or redirects, for example.

The Document Object Model is the representation of a web resource's structure by the browser defined by a set of HTML tags, each called a node. The DOM can be dynamically manipulated following the modification, addition, or deletion of nodes in Javascript.

The fallback is a workaround solution that involves offering an alternative in case a feature or resource proves incompatible with the user's browser, or if it becomes inaccessible.

The First Contentful Paint is a performance metric that measures the time it takes for the first visible element of the page to be displayed. This can be text, an image, or even a simple HTML element. The FCP is heavily impacted by blocking resources such as synchronous scripts or stylesheets. It is the first front-end metric and is the step following TTFB in the lifecycle of a web page.

The First Input Delay is a performance metric that measures the time it takes before a page is considered to be interactive smoothly. It is part of the 3 Core Web Vitals but will be replaced by INP in 2024 for better consideration of the UX of websites. A good FID is less than 100ms, and it is poor when it exceeds 300ms. This metric will be replaced by INP in 2024.

The front-end is the accessible and visible part of a website for its visitors. In contrast to the back-end, it is managed by the browser and can be manipulated by resources loaded by the site, third parties, or even extensions installed in the user's browser.

A developer is said to be full-stack when they master the technical environments end-to-end on the server side (back-end languages like PHP or Java, SQL syntax for databases) as well as on the browser side (HTML, CSS, Javascript, …).

Compression algorithm used by web servers to reduce the size of text resources (html, CSS, JS, …). This algorithm is older than Brotli and offers a less significant reduction in the size of resources, but is decompressed more quickly.

The .htaccess and .htpasswd files are accessed by the Apache web server before serving any resource or web page to the user. These files allow for instructions such as URL rewriting (to remove file extensions like .php or .html, for example), as well as defining cache durations for users. The .htpasswd file is used to protect access to a folder with a login/password prompt.

The HTML language, or HyperText Markup Language, is a language used to define the structure of a web page. Being not a programming language, it consists of a large number of tags, each serving a different purpose. HTML tags include paragraphs, images, tables, videos… The entire HTML code of a web page is called the DOM.

The Iframe is an HTML element that allows you to display a web page within another document. The Iframe can be displayed with or without a border, in defined dimensions but cannot modify the document in which it is displayed. It is an HTML tag often used by advertising publishers.

Inline code is inserted directly within the DOM of the HTML page. Unlike an external file called by the document, inline code is never cached. The most common inline tags are

INP is the new web performance metric in beta since 2022, set to replace FID in March 2024. Acronym for “Interaction to Next Paint,” this performance indicator measures a page's responsiveness following a user interaction. INP is considered good under 200 milliseconds (the threshold deemed imperceptible by the human eye), and it is considered poor beyond 500 milliseconds.