css_inline is a high-performance library for inlining CSS into HTML 'style' attributes.
This library is designed for scenarios such as preparing HTML emails or embedding HTML into third-party web pages.
For instance, the library transforms HTML like this:
<html>
<head>
<style>h1 { color:blue; }</style>
</head>
<body>
<h1>Big Text</h1>
</body>
</html>
into:
<html>
<head></head>
<body>
<h1 style="color:blue;">Big Text</h1>
</body>
</html>
style and link tagsstyle and link tagsIf you'd like to try css-inline, you can check the WebAssembly-powered playground to see the results instantly.
Install with pip:
pip install css_inline
Pre-compiled wheels are available for most popular platforms. If not available for your platform, a Rust compiler will be needed to build this package from source. Rust version 1.65 or higher is required.
import css_inline
HTML = """<html>
<head>
<style>h1 { color:blue; }</style>
</head>
<body>
<h1>Big Text</h1>
</body>
</html>"""
inlined = css_inline.inline(HTML)
# HTML becomes this:
#
# <html>
# <head>
# <style>h1 { color:blue; }</style>
# </head>
# <body>
# <h1 style="color:blue;">Big Text</h1>
# </body>
# </html>
Note that css-inline automatically adds missing html and body tags, so the output is a valid HTML document.
Alternatively, you can inline CSS into an HTML fragment, preserving the original structure:
FRAGMENT = """<main>
<h1>Hello</h1>
<section>
<p>who am i</p>
</section>
</main>"""
CSS = """
p {
color: red;
}
h1 {
color: blue;
}
"""
inlined = css_inline.inline_fragment(FRAGMENT, CSS)
# HTML becomes this:
# <main>
# <h1 style="color: blue;">Hello</h1>
# <section>
# <p style="color: red;">who am i</p>
# </section>
# </main>
When there is a need to inline multiple HTML documents simultaneously, css_inline offers inline_many and inline_many_fragments functions.
This feature allows for concurrent processing of several inputs, significantly improving performance when dealing with a large number of documents.
import css_inline
css_inline.inline_many(["<...>", "<...>"])
Under the hood, inline_many, spawns threads at the Rust layer to handle the parallel processing of inputs.
This results in faster execution times compared to employing parallel processing techniques at the Python level.
Note: To fully benefit from inline_many, you should run your application on a multicore machine.
For configuration options use the CSSInliner class:
import css_inline
inliner = css_inline.CSSInliner(keep_style_tags=True)
inliner.inline("...")
inline_style_tags. Specifies whether to inline CSS from "style" tags. Default: Truekeep_style_tags. Specifies whether to keep "style" tags after inlining. Default: Falsekeep_link_tags. Specifies whether to keep "link" tags after inlining. Default: Falsebase_url. The base URL used to resolve relative URLs. If you'd like to load stylesheets from your filesystem, use the file:// scheme. Default: Noneload_remote_stylesheets. Specifies whether remote stylesheets should be loaded. Default: Truecache. Specifies caching options for external stylesheets (for example, StylesheetCache(size=5)). Default: Noneextra_css. Extra CSS to be inlined. Default: Nonepreallocate_node_capacity. Advanced. Preallocates capacity for HTML nodes during parsing. This can improve performance when you have an estimate of the number of nodes in your HTML document. Default: 32You can also skip CSS inlining for an HTML tag by adding the data-css-inline="ignore" attribute to it:
<head>
<style>h1 { color:blue; }</style>
</head>
<body>
<!-- The tag below won't receive additional styles -->
<h1 data-css-inline="ignore">Big Text</h1>
</body>
The data-css-inline="ignore" attribute also allows you to skip link and style tags:
<head>
<!-- Styles below are ignored -->
<style data-css-inline="ignore">h1 { color:blue; }</style>
</head>
<body>
<h1>Big Text</h1>
</body>
Alternatively, you may keep style from being removed by using the data-css-inline="keep" attribute.
This is useful if you want to keep @media queries for responsive emails in separate style tags:
<head>
<!-- Styles below are not removed -->
<style data-css-inline="keep">h1 { color:blue; }</style>
</head>
<body>
<h1>Big Text</h1>
</body>
Such tags will be kept in the resulting HTML even if the keep_style_tags option is set to false.
If you'd like to load stylesheets from your filesystem, use the file:// scheme:
import css_inline
# styles/email is relative to the current directory
inliner = css_inline.CSSInliner(base_url="file://styles/email/")
inliner.inline("...")
You can also cache external stylesheets to avoid excessive network requests:
import css_inline
inliner = css_inline.CSSInliner(
cache=css_inline.StylesheetCache(size=5)
)
inliner.inline("...")
Caching is disabled by default.
If you'd like to work around some XHTML compatibility issues like closing empty tags (<hr> vs. <hr/>), you can use the following snippet that involves lxml:
import css_inline
from lxml import html, etree
document = "..." # Your HTML document
inlined = css_inline.inline(document)
tree = html.fromstring(inlined)
inlined = etree.tostring(tree).decode(encoding="utf-8")
css-inline is powered by efficient tooling from Mozilla's Servo project and significantly outperforms other Python alternatives in terms of speed.
Most of the time it achieves over a 10x speed advantage compared to the next fastest alternative.
Here is the performance comparison:
| Size | css_inline 0.14.1 | premailer 3.10.0 | toronado 0.1.0 | inlinestyler 0.2.5 | pynliner 0.8.0 | |
|---|---|---|---|---|---|---|
| Basic | 230 B | 6.54 µs | 127.62 µs (19.51x) | 657.50 µs (100.52x) | 1.02 ms (157.01x) | 1.17ms (179.64x) |
| Realistic-1 | 8.58 KB | 134.54 µs | 1.40 ms (10.42x) | 15.81 ms (117.54x) | 26.37 ms (196.04x) | 52.77 ms (392.29x) |
| Realistic-2 | 4.3 KB | 82.37 µs | 2.78 ms (33.80x) | ERROR | 17.71 ms (215.01x) | ERROR |
| GitHub page | 1.81 MB | 223.85 ms | 25.04 s (111.90x) | ERROR | ERROR | ERROR |
The "Basic" case was obtained by benchmarking the example from the Usage section.
Note that the toronado, inlinestyler, and pynliner libraries both encountered errors when used to inline CSS in the last scenario.
The benchmarking code is available in the benches/bench.py file. The benchmarks were conducted using the stable rustc 1.78, Python 3.11.7 on M1 Max.
Besides performance, css-inline differs from other Python libraries for CSS inlining.
toronado and pynliner do not support pseudo-elements);premailer;premailer, inlinestyler, toronado, and optionally pynliner);If you want to know how this library was created & how it works internally, you could take a look at these articles:
This project is licensed under the terms of the MIT license.
FAQs
High-performance library for inlining CSS into HTML 'style' attributes
We found that css-inline demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 1 open source maintainer collaborating on the project.
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