A/B testing

Test consisting of creating alternative versions of a single page, by making a feature vary (content, overall aspect, presentation…), in order to measure the impacts (transformation rate, turnover…) and then to determine the most powerful version.


Invisible part of a website made of several elements such as: a HTTP server (e .g. Apache), a PHP application (WordPress, PrestaShop, etc.), a database (MySql), etc.


This notion, applied to your web server (your host), is the maximum speed of data that it can receive. This notion is important for the sites with a lot of traffic or the sites being big data consumers (streaming) because the bandwidth can constitute a bottleneck (the amount of data to a given moment is over the channel capacity).

Basis 64

Encoding of the information using 64 textual characters. Maybe used to directly include resources (images for instance) in the HTML code, avoiding to make extra HTTP requests to download them.

Byte Rate

Measure of the quantity of transmitted data per time unit (given in Megabits/second for instance).


Temporary saving of a resource. There are two types of cache: the client cache and the server cache. The client cache allows to save the data (e.g. an image) directly on the web browser. This avoids to download the resource again later and to speed up the downloading. The server cache consists in saving a calculation, a processing, a page on the server in order to speed up its availability to the users.


Network of servers delivering identical contents, located throughout the world. This way, the contents will be sent to the user from the closest server, in order to reduce the latency and the global loading time of the page.

Critical rendering path

List of operations your Internet browser must do (downloading, interpreting, processing…) to display your web page. Optimizing this path enables to speed the display of the page and to prioritize important contents.

CSS Sprite

Method that consists in gathering several images in a single file in order to cut the number of HTTP requests. The display of each image within your page is then made by the application of CSS styles to assure the cutting of the global image.


Operation that allows cutting the amount of data of a file or a communication. On a web communication, a resource is compressed (gzip) by the server then transits through the network and is decompressed before its interpretation by the user's browser.


Hierarchy representing a HTML document, gathered and used by the web browser in order to represent the structure of a web page.

Execution of JavaScript

Consists in interpreting the instructions of a JavaScript file. The execution of a JavaScript file is blocking, which means that the execution of the script must be over so that the page finishes loading/displaying its content. The asynchronous execution enables to postpone the treatments, and then to speed up the display of the page.


Line above which the content of the web page is visible without scrolling the page. The part above this line should be your point of interest when optimizing the critical rendering path.


Elements of the website that are downloaded by the web browser, and that they are able to interact with (HTML, CSS, JavaScript…). It's the « visible » part of the site.

HTTP request

Request carried out via the HTTP protocol by the web browser to a server in order to get a web resource (HTML, JavaScript, CSS…) or in order to trigger an action.


Hyper Text Transfer Protocol Secure. It is the secured version of the HTTP protocol that enables the communication between a browser and a web server. HTTPS consists in encoding theses communications to assure their integrity and their confidentiality.


Incompressible delay required so that the data go from A (e.g. your home) to B (e.g. the web server). The latency is related to the distance between those two points.


Loading of the elements upon request, meaning during the user's interaction. For example: only loading the visible images to the screen. The images located further in the web page will be downloaded only if the user scrolls down the page.


Method that deletes every useless characters to the interpretation of the code by a web browser, meaning: spaces, tabs, line feeds, or else comments. The characters are useful for the developers, but are not for the browsers. Deleting them enables to reduce the quantity of data and then to speed up the web page.

Performance timings

Main steps of a page loading time (server response time, interaction time with the page, total loading time…) and of which the measures are provided by the web browsers.

RUM (Real User Monitoring)

Performance monitoring made by capturing and measuring the data (for example the Performance Timings) directly on the traffic (or sample of the traffic) of a web site.

Speed Index

Performance index that transcribes the display speed of the visible part (above the waterline) of a web page, by taking into account the progressiveness of the display of many components. Reference point: if the page is completely displayed in 1 second, the Speed Index is equal to 1000.

Third Party

External element used on a web page, provided by a server that the owner of the website in consideration (e.g. Google Analytics) does not administer. The stakes are important because the person in charge of the site has no grip on the availability, the reliability or else the performance of this third party.

Time To First Byte

Time elapsed between the sending of the request requiring the web page and the reception of the first data by the user. The TTFB is particularly impacted by the latency.

Time To Interact

Time elapsed between the sending of the request by the web browser and the moment the user is able to interact with a component of the page (e.g. search bar). This component must be displayed and the web browser must respond to the user's actions (a processing shouldn't block it).

Visually Complete

Minimum time needed for the zone above the fold to be rendered under its final form. So that means the part of the page available without any action from the user.

W3C Validator

Tools allowing to validate the HTML and the CSS regarding the standards defined by the W3C (that edits the specifications of the number of web technologies).

Web server

Machine and/or application that is part of the backend and that provides HTTP requests with responses and that implements the related actions. The web server provides to the web browsers the required resources to the display of the requested page.