The cache manifest in HTML5 was a software storage feature which provided the ability to access a web application even without a network connection. It became part of the W3C Recommendation on 28 October 2014.
Since 2021, this technology is no longer widely available. It was removed from Firefox 85, and disabled by default in Chrome 84 and removed in Chrome 95. Using any of the offline Web application features at this time is highly discouraged and use of service workers is recommended instead. Cache manifests are distinct from web application manifests, a JSON-based file format which is part of the progressive web app technology, and is currently active and going through the standardization process at the W3C.
Background
Web applications consist of web pages that need to be downloaded from a network. For this to happen there must be a network connection. However, there are many instances when users cannot connect to a network due to circumstances beyond their control. HTML5 provides the ability to access the web application even without a network connection using the cache manifest.
Web applications consist of resources identified by URLs. These can be HTML, CSS, JavaScript, images or any other source that is required for a web application to be rendered. Their addresses can be copied into a manifest file, which can be updated regularly by the author of the web application, indicating any new web addresses that are added or deleted. When connecting to a network for the first time, a web browser will read the HTML5 manifest file, download the resources given and store them locally. Then, in the absence of a network connection, the web browser will shift to the local copies instead and render the web application offline.
Basics
In order for the offline applications to work, a cache manifest file must be created by the web developer. If the web application exceeds more than one page then each page must have a manifest attribute that points to the cache manifest. Every page referencing the manifest will be stored locally. The cache manifest file is a text file located in another part of the server. It must be served with content type text/cache-manifest
The attribute manifest="<path>" must be added to the html element in order for the cache manifest file to work. Example: <!DOCTYPE HTML> <html manifest="cache.appcache"> <body> … </body> </html>
The argument to the manifest attribute is a relative or absolute path to the manifest file.
Consider the HTML file given below. The <html> element indicates a file named cache.appcache will contain the list of resources (i.e., test.js, test.css) needed for this web page to work offline. Common names for this file are cache.manifest and manifest.appcache. <!—- test.html --> <!DOCTYPE HTML> <html manifest="cache.appcache"> <head> <title>Test</title> <script src="test.js"></script> <link rel="stylesheet" href="test.css"> </head> <body> Testing the manifest file. </body> </html>
Syntax
The manifest file must start with the line CACHE MANIFEST. Comments start with a #, spaces and blank lines are ignored.
Given below is an example of a cache manifest file.
Example 1: CACHE MANIFEST /test.css /test.js /test.png
This manifest file lists three resources: a CSS file, a JavaScript file and a PNG image. When the above file is loaded, the browser will download the test.css, test.js and test.png files from the root directory in the web server. As a result, whenever one's network is not connected, the resources will be available to them offline.
Cache manifests can also use relative paths or even absolute URLs as shown below.
Example 2: CACHE MANIFEST /main/features.js /main/settings/index.css http://files/images/scene.jpg http://files/images/world.jpg
File headers
The cache manifest file consists of three section headers. Explicit section with the header CACHE. Online whitelist section with the header NETWORK. Fallback section with the header FALLBACK.
Note: Example 1 and Example 2 above, do not indicate any section header and are therefore considered an explicit section by default.
Online whitelist section with the header NETWORK
Example 3: CACHE MANIFEST NETWORK: /checking.cgi CACHE: /test.css /test.js /test.png
This example consists of headers. The line, NETWORK: is the start of the "online whitelist" section. The resources listed under this section are never cached and are not available offline. As a result, an error will occur when an attempt is made offline to load the resource.