An extension is simply the fact of increasing the size of a content on a WEB page. When a user navigates on a site and more particularly from a phone, he may not be able to read the text or wants to look more precisely at an image, in this case, he will zoom in on the page to obtain a suitable size.
It is possible to prohibit the extension of a WEB page via the "meta viewport" tag, which gives instructions on the size and scale of the elements depending on the browser window. When this prohibition is set, the user will not be able to zoom in on the content and may not even be able to read the text on the page. This frustration degrades the user experience and may cause the user to run away.
By default, browsers allow zooming, if no "meta viewport" tag is defined, there is no need to intervene.
If a "meta viewport" tag is defined, make sure that :
Example of a "meta viewport" prohibiting enlargement :
<meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no" />
Example of a "meta viewport" allowing enlargement :
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=6" />