Critical extracts & inlines critical-path (above-the-fold) CSS from HTML
$ npm install --save critical
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var critical = ;
Full blown example with available options:
Generate and inline critical-path CSS
Generate critical-path CSS
Generate and minify critical-path CSS:
Generate, minify and inline critical-path CSS:
Generate and return output via callback:
Generate and return output via promise:
Generate critical-path CSS with multiple resolutions
When your site is adaptive and you want to deliver critical CSS for multiple screen resolutions this is a useful option. note: (your final output will be minified as to eliminate duplicate rule inclusion)
Generate critical-path CSS and ignore specific selectors
This is a useful option when you e.g. want to defer loading of webfonts or background images.
||Inline critical-path CSS using filamentgroup's loadCSS. Pass an object to configure
||Base directory in which the source and destination are to be written|
||HTML source to be operated against. This option takes precedence over the
||HTML source folder. Required to compute relative asset paths in conjunction with the
||An array of paths to css files, or an array of Vinyl file objects.|
||Location of the HTML source to be operated against|
||Location of where to save the output of an operation (will be relative to base if no absolute path is set)|
||Subfolder relative to base directory. Only relevant without src (if raw html is provided) or if the destination is outside base|
||Target file to store the generated critical-path styles|
||Width of the target viewport|
||Height of the target viewport|
||An array of objects containing height and width. Takes precedence over
||Enable minification of generated critical-path CSS|
||Remove the inlined styles from any stylesheets referenced in the HTML. It generates new references based on extracted content so it's safe to use for multiple HTML files referencing the same stylesheet. Use with caution. Removing the critical CSS per page results in a unique async loaded CSS file for every page. Meaning you can't rely on cache across multiple pages|
||List of directories/urls where the inliner should start looking for assets|
||Sets a max file size (in bytes) for base64 inlined images|
||Sets a maximum timeout for the operation|
||Path to prepend CSS assets with. You must make this path absolute if you are going to be using critical in multiple target files in disparate directory depths. (eg. targeting both
||Force include CSS rules. See
||Ignore CSS rules. See
||Ignore options. See
||User agent to use when fetching a remote src|
||Configuration options for
||RFC2617 basic authorization: user|
||RFC2617 basic authorization: pass|
$ npm install -g critical
critical works well with standard input.
$ cat test/fixture/index.html | critical --base test/fixture --inline > index.critical.html
You can also pass in the critical CSS file as an option.
$ critical test/fixture/index.html --base test/fixture > critical.css
var gulp = ;var log = ;var critical = stream;// Generate & Inline Critical-path CSSgulp;
Why is critical-path CSS important?
Why should critical-path CSS be inlined?
For best performance, you may want to consider inlining the critical CSS directly into the HTML document. This eliminates additional roundtrips in the critical path and if done correctly can be used to deliver a “one roundtrip” critical path length where only the HTML is a blocking resource.
Are there any sample projects available using Critical?
Why, yes!. Take a look at this Gulp project which demonstrates using Critical to generate and inline critical-path CSS. It also includes a mini-tutorial that walks through how to use it in a simple webapp.
When should I just use Penthouse directly?
The main differences between Critical and Penthouse, a module we use, are:
- Critical will automatically extract stylesheets from your HTML from which to generate critical-path CSS from, whilst other modules generally require you to specify this upfront.
- Critical provides methods for inlining critical-path CSS (a common logical next-step once your CSS is generated)
- Since we tackle both generation and inlining, we're able to abstract away some of the ugly boilerplate otherwise involved in tackling these problems separately.
That said, if your site or app has a large number of styles or styles which are being dynamically injected into the DOM (sometimes common in Angular apps) I recommend using Penthouse directly. It will require you to supply styles upfront, but this may provide a higher level of accuracy if you find Critical isn't serving your needs.
What other alternatives to Critical are available?
Is Critical stable and suitable for production use?
Critical has been used on a number of production sites that have found it stable for everyday use. That said, we welcome you to try it out on your project and report bugs if you find them.
Can I contribute?
Of course. We appreciate all of our contributors and welcome contributions to improve the project further. If you're uncertain whether an addition should be made, feel free to open up an issue and we can discuss it.
This module is brought to you and maintained by the following people:
Apache-2.0 © Addy Osmani, Ben Zörb