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Java CSS 2 and CSS 3 parser and builder. This version supersedes phloc-css. The Maven plugin to compress CSS files at build time is located in sub-project ph-csscompress-maven-plugin and described further down.

ph-css has no logic for applying CSS onto HTML elements. This page shows some basic code examples that can be used to use the library. All snippets are free for any use.

ph-css and ph-csscompress-maven-plugin are both licensed under the Apache 2.0 license.

ph-css is used as a part of Apache JMeter 3 :)

Maven usage

Add the following to your pom.xml to use this artifact, replacing x.y.z with the latest version:


To build ph-css from source, Maven 3.0.4 is required. Any Maven version below does NOT work!


As ph-css is mainly concerned about the grammatical structure of CSS, the main classes are for reading and writing CSS. Additionally it offers the possibility to traverse the elements in a CSS and make modifications on them.

Coding paradigms used

Please look at my personal Coding Styleguide for the naming conventions used in this project.

Basic Classes

A complete stylesheet is represented as an instance of com.helger.css.decl.CascadingStyleSheet. There is no difference between CSS 2.1 and CSS 3.0 instances. The class com.helger.css.decl.CascadingStyleSheet contains all top-level rules that may be present in a CSS:

  • Import rules (@import) - com.helger.css.decl.CSSImportRule
  • Namespace rules (@namespace) - com.helger.css.decl.CSSNamespaceRule
  • Style rules (e.g. div{color:red;}) - com.helger.css.decl.CSSStyleRule
  • Page rules (@page) - com.helger.css.decl.CSSPageRule
  • Media rules (@media) - com.helger.css.decl.CSSMediaRule
  • Font face rules (@font-face) - com.helger.css.decl.CSSFontFaceRule
  • Keyframes rules (@keyframes) - com.helger.css.decl.CSSKeyframesRule
  • Viewport rules (@viewport) - com.helger.css.decl.CSSViewportRule
  • Supports rules (@supports) - com.helger.css.decl.CSSSupportsRule
  • Any other unknown rules (@foo) - com.helger.css.decl.CSSUnknownRule

CSS reading

ph-css contains two different possibilities to read CSS data:

  • Reading a complete CSS file can be achieved using com.helger.css.reader.CSSReader. The result in this case will be an instance of com.helger.css.decl.CascadingStyleSheet.
  • Reading only a list of style information (as e.g. present in an HTML style element) can be achieved using com.helger.css.reader.CSSReaderDeclarationList. The result in this case will be an instance of com.helger.css.decl.CSSDeclarationList.

Both reading classes support the reading from either a, a, a or a String. The reason why is not supported directly is because internally the stream is passed twice - first to determine a potentially available charset and second to read the content with the correctly determined charset. That's why an IInputStreamProvider must be used, that creates 2 unique input streams!

Note: reading from a String is possible in two different ways: one that requires a charset and one that doesn't. The version with the charset treats the String as if it was created from a byte array and tries to determine the charset like any other byte array based version. The version without a charset assumes that the String was already created with the correct charset, and any @charset rule contained in the CSS is ignored.

Since v3.8.2 the class com.helger.css.reader.CSSReaderSettings is present and encapsulates the CSS version, the fallback charset, the recoverable error handler (see below) and the unrecoverable error handler (also see below) in one settings object. This settings object can be used for multiple invocations of CSSReader and CSSReaderDeclarationList.

Recoverable Errors

ph-css differentiates between recoverable errors and unrecoverable errors. An example for a recoverable error is e.g. an @import rule in the wrong place or a missing closing bracket within a style declaration. For recoverable errors a special handler interface com.helger.css.reader.errorhandler.ICSSParseErrorHandler is present. You can pass an implementation of this error handler to the CSS reader (see above). The following implementations are present by default (all residing in package com.helger.css.reader.errorhandler):

  • DoNothingCSSParseErrorHandler - silently ignoring all recoverable errors
  • LoggingCSSParseErrorHandler - logging all recoverable errors to an SLF4J logger
  • ThrowingCSSParseErrorHandler - throws a com.helger.css.parser.ParseException in case of a recoverable error which is afterwards handled by the unrecoverable error handler (see below). This can be used to enforce handling only 100% valid CSS files. This is the default setting, if no error handler is specified during reading.
  • CollectingCSSParseErrorHandler - collects all recoverable errors into a list of com.helger.css.reader.errorhandler.CSSParseError instances for later evaluation.

Some error handlers can be nested so that a combination of a logging handler and a collecting handler can easily be achieved like:

new CollectingCSSParseErrorHandler (new LoggingCSSParseErrorHandler ())

DoNothingCSSParseErrorHandler and ThrowingCSSParseErrorHandler cannot be nested because it makes no sense.

Both CSSReader and CSSReaderDeclarationList have the possibility to define a default recoverable error handler using the method setDefaultParseErrorHandler(ICSSParseErrorHandler). If a reading method is invoked without an explicit ICSSParseErrorHandler than this default error handler is used.

Unrecoverable Errors

In case of an unrecoverable error, the underlying parser engine of JavaCC throws a com.helger.css.parser.ParseException. This exception contains all the necessary information on where the error occurred. In case of such an unrecoverable error, the result of the reading will always be null and the exception is not automatically propagated to the caller. To explicitly get notified when such a parse error occurs, the handler interface com.helger.css.handler.ICSSParseExceptionCallback is available. The available implementations are (all residing in package com.helger.css.handler):

  • DoNothingCSSParseExceptionHandler - silently ignore all unrecoverable errors
  • LoggingCSSParseExceptionHandler - log all unrecoverable errors to an SLF4J logger

As there is at most one unrecoverable error per parse there is no collecting implementation of an ICSSParseExceptionCallback available. If it is desired to propagate the Exception to the caller you need to implement your own ICSSParseExceptionCallback subclass that throws an unchecked exception (one derived from RuntimeException). Example:

  final ICSSParseExceptionCallback aThrowingExceptionHandler = new ICSSParseExceptionCallback () {
    public void onException (final ParseException ex) {
      throw new IllegalStateException ("Failed to parse CSS", ex);

Both CSSReader and CSSReaderDeclarationList have the possibility to define a default unrecoverable error handler using the method setDefaultParseExceptionHandler(ICSSParseExceptionCallback). If a reading method is invoked without an explicit ICSSParseExceptionCallback than this default exception handler is used.

CSS iteration/visiting

Once a CSS file was successfully read, it can easily be iterated using the class com.helger.css.decl.visit.CSSVisitor. It requires a valid instance of com.helger.css.decl.CascadingStyleSheet as well as an implementation of com.helger.css.decl.visit.ICSSVisitor. The CascadingStyleSheet can be acquired either by reading from a file/stream or by creating a new one from scratch. For the ICSSVisitor it is recommended to use the class com.helger.css.decl.visit.DefaultCSSVisitor as the base class - this class contains empty implementations of all methods defined in the ICSSVisitor interface. To visit all declarations (e.g. color:red;) it is sufficient to simply override the method public void onDeclaration (@Nonnull final CSSDeclaration aDeclaration). For details please have a look at the JavaDocs of ICSSVisitor. To start the visiting call CSSVisitor.visitCSS (CascadingStyleSheet, ICSSVisitor).

A special visitor is present for URLs. URLs can occur on several places in CSS files, especially in the @import rules and within declarations (like in background-image: url(../images/bg.gif)). Therefore a special interface com.helger.css.decl.visit.ICSSUrlVisitor together with the empty default implementation com.helger.css.decl.visit.DefaultCSSUrlVisitor is provided. So to visit all URLs within a CSS call CSSVisitor.visitCSSUrl(CascadingStyleSheet, ICSSUrlVisitor). Since v6.2.3 the URL visitor recursively descends into expression members (see issue #59).

For modifying URLs (e.g. to adopt paths to a different environment) a special base class com.helger.css.decl.visit.AbstractModifyingCSSUrlVisitor is available. It offers the abstract method protected abstract String getModifiedURI (@Nonnull String sURI) to modify a URL and write the result back into the original CascadingStyleSheet. An example of how this can be used, can be found in the test method com.helger.css.decl.visit.CSSVisitorDeclarationListTest.testModifyingCSSUrlVisitor ().

Note: it is safe to modify a CSS while iterating it, but only changes affecting children of the current node may be considered during the same iteration run.

CSS writing

CSS writing is performed with the class com.helger.css.writer.CSSWriter. The most basic settings can be passed either directly to the constructor or using an instance of com.helger.css.writer.CSSWriterSettings which offers a quite find grained control of the output process. To write the content of a CascadingStyleSheet or any ICSSWriteable to an arbitrary, the method writeCSS is what you need. If you want the CSS serialized to a String the shortcut method getCSSAsString is available. For the remaining configuration methods please check the JavaDoc.

By default all CSS code is pretty-printed. To create a minified version of the CSS code call setOptimizedOutput (true) and setRemoveUnnecessaryCode (true) on your CSSWriterSettings object.

Data URL handling

Data URLs are URLs that directly contain the content inline. A regular use case is referencing small images directly inside a CSS. During CSS parsing no special handling for data URLs is added. Instead they are stored in a String like any other URL.

To special handle data URLs the class com.helger.css.utils.CSSDataURLHelper offers the possibility to check if a URL is a data URL via public static boolean isDataURL (String). If this method returns true the method public static CSSDataURL parseDataURL (String) can be used to extract all the information contained in the data URL. This method returns null if the passed URL is not a data URL.

Shorthand property handling

A CSS shorthand property is a property that consists of multiple values. Classical examples are margin or border. ph-css contains support for selected shorthand properties. All shorthand related classes can be found in package com.helger.css.decl.shorthand. The supported shorthand properties are:

  • background
  • font
  • border
  • border-top
  • border-right
  • border-bottom
  • border-left
  • border-width
  • border-style
  • border-color
  • margin
  • padding
  • outline
  • list-style

All of these shorthand properties are registered in class CSSShortHandRegistry and you can manually register your own shorthand descriptors. The CSSShortHandRegistry allows you to split a single CSSDeclaration like border:1px dashed into the corresponding "sub-declarations":

  // Parse a dummy declaration
  final CSSDeclaration aDecl = CSSReaderDeclarationList.readFromString ("border:1px dashed", ECSSVersion.CSS30).getDeclarationAtIndex (0);

  // Get the Shorthand descriptor for "border"    
  final CSSShortHandDescriptor aSHD = CSSShortHandRegistry.getShortHandDescriptor (ECSSProperty.BORDER);

  // And now split it into pieces
  final List <CSSDeclaration> aSplittedDecls = aSHD.getSplitIntoPieces (aDecl);

In the above example, aSplittedDecls will contain 3 elements with the following content:

  • border-width:1px
  • border-style:dashed
  • border-color:black

Even though no color value was provided, the default value black is returned. For all "sub-declarations", sensible default values are defined.

CSS utilities

ph-css contains a multitude of small utility class covering different aspects of CSS

  • com.helger.css.utils.CSSColorHelper contains methods to read and write the different types of CSS color values (rgb, rgba, hsl, hsla and hex value)
  • com.helger.css.utils.ECSSColor contains the basic CSS colors as an enumeration
  • com.helger.css.ECSSUnit contains all the default CSS units (like. px or em)
  • com.helger.css.utils.CSSNumberHelper contains methods for handling the combination of numeric values and units.
  • com.helger.css.utils.CSSRectHelper contains methods for handling CSS rect values.
  • provides shortcut methods for wrapping a complete CascadingStyleSheet in one or more media queries

Code Examples

  • Reading a CSS 3.0 file
  • Writing a CSS 3.0 file
  • Creating a @font-face rule from scratch
  • The code creates a CSS @font-face rule that looks like this:
    @font-face {
      font-family: "Your typeface";
      src: url("path/basename.eot");
      src: local("local font name"),
           url("path/basename.woff") format("woff"),
           url("path/basename.otf") format("opentype"),
           url("path/basename.svg#filename") format("svg");
  • Read the CSS content of a HTML style attribute
    • Reads the CSS content of sStyle as CSS 3.0 and creates a CSSDeclarationList from it
  • Visiting all declarations contained in an HTML style attribute
    • Similar to the above example, but visiting all declarations and printing them on stdout. Two different approaches are shown: first all declarations are retrieved via the native API, and second a custom visitor is used to determine all declarations. The result of this method looks like this:
    color: red (not important)
    background: fixed (important)
  • Visit all URLs contained in a CSS
    • Read a CSS from a String and than extracts all contained URLs. The output looks like this:
    Import: foobar.css - source location reaches from [1/1] up to [1/21]
    background - references: a.gif - source location reaches from [2/22] up to [2/31]
    background-image - references: /my/folder/b.gif - source location reaches from [3/25] up to [3/47]
  • Visit all URLs ( URLs) contained in a CSS
    • Read a CSS from a String and than extracts all contained URLs with special focus on data URLs. The output looks like this:
    Import: /folder/foobar.css
    background - references data URL with 158 bytes of content
    background-image - references regular URL: /my/folder/b.gif

Known shortcomings

The following list gives an overview of known shortcomings in ph-css

  • Escaped characters (like \26) are not interpreted correctly.


A Maven plugin to compress CSS files at build time using ph-css.

It requires Java 8 and Maven 3 to run.

Maven configuration

Replace x.y.z with the version you want to use.


Configuration items are:

  • File sourceDirectory
    The directory where the CSS files reside. It must be an existing directory.
    Defaults to ${basedir}/src/main/resources
  • boolean recursive
    Should all directories be scanned recursively for CSS files to compress? Defaults to true
  • boolean removeUnnecessaryCode
    Should unnecessary code be removed (e.g. rules without declarations)? Defaults to false
  • boolean quoteURLs
    Should URLs always be quoted? If false they are only quoted when absolutely necessary. Defaults to false
  • boolean writeNamespaceRules
    Should @namespace rules be written? Defaults to true
  • boolean writeFontFaceRules
    Should @font-face rules be written? Defaults to true
  • boolean writeKeyframesRules
    Should @keyframes rules be written? Defaults to true
  • boolean writeMediaRules
    Should @media rules be written? Defaults to true
  • boolean writePageRules
    Should @page rules be written? Defaults to true
  • boolean writeViewportRules
    Should @viewport rules be written? Defaults to true
  • boolean writeSupportsRules
    Should @supports rules be written? Defaults to true
  • boolean writeUnknownRules
    Should unknown @ rules be written? Defaults to true
  • boolean forceCompress
    Should the CSS files be compressed, even if the timestamp of the compressed file is newer than the timestamp of the original CSS file? Defaults to false
  • boolean verbose
    If true some more output is emitted. Defaults to false
  • boolean browserCompliantMode (since 1.4.0) If true the "browser compliant mode" for parsing is selected. Defaults to false.
  • String sourceEncoding
    The encoding of the source CSS files to be used for reading the CSS file in case neither a @charset rule nor a BOM is present. Defaults to UTF-8
  • String targetFileExtension
    The filename extension that should be used for the minified/compressed CSS file. Defaults to .min.css
  • String targetEncoding (since 1.4.0) The encoding of the target CSS files to be used for writing the CSS file. Defaults to UTF-8.

News and noteworthy

  • v6.3.4 - 2021-06-09
    • Fixed an error with the shorthand expansion of border-width (PR #68) - thanks @rockwotj
  • v6.3.3 - 2021-05-31
  • v6.3.2 - 2021-05-25
  • v6.3.1 - 2021-05-02
  • v6.3.0 - 2021-03-22
    • Updated to ph-commons 10
  • v6.2.4 - 2021-02-25
    • Updated to ph-commons 9.4.6, leading to a more resilient DataURL parsing
    • Fixed the ignorance of minimum and maximum parameter count in class CSSPropertyColors
    • Fixed the case-sensitivity of CSS variables declarations (issue #63)
  • v6.2.3 - 2020-05-14
    • Fixed an issue with the URL visitor not recursively descending into expression members (issue #59)
  • v6.2.2 - 2020-03-29
    • Updated to ph-commons 9.4.0
  • v6.2.1 - 2020-02-29
    • Improved the pattern for colors in hex values (PR #55)
    • Fixed a grammar issue that resulted in failed calc parsing (issue #57). The implications of this change are, that the special IE6 and IE7 hacks with $ and * as identifier prefixes are no longer supported. If you need to parse CSS suitable for these old browsers, stay with v6.2.0.
    • Updated CSS properties of the completed specifications
  • v6.2.0 - 2019-06-30
    • Fixed NPE in CSSWriterSettings.DEFAULT_SETTINGS because of wrong initialization order (#53)
    • Added some -o- media expression features
    • Changing browser compliant parsing in a way, so that less null is returned in parsing (#41)
    • The default "custom error handler" for parsing was changed from ThrowingCSSParseErrorHandler to LoggingCSSParseErrorHandler for best browser compliant mode handling
    • Improved fault tolerance in parsing when "browser compliant mode" is enabled
  • v6.1.3 - 2019-06-12
    • Added some performance tweaks
    • Added possibility to disable the usage of the source location using CSSReaderSettings (for performance reasons)
  • v6.1.2 - 2019-02-17
    • Extended list of known media expression features (e.g. used in Bootstrap 4)
  • v6.1.1 - 2018-04-11
    • Allow nested calc function (#48)
  • v6.1.0 - 2018-03-23
    • Fixed a grammar issue for calc with spaces (as in width: calc( 100% - 2.4em );)
    • Removed dependency upon java.awt.Color for improved Android compatibility.
  • v6.0.0 - 2017-12-20
    • Extracted base class AbstractHasTopLevelRules for type-safe access of top-level rules (#39)
    • Updated to ph-commons 9.0.0
    • Extended API to more easily access the CSS declaration expression string (#43)
    • Data URI scheme is treated case-insensitive (#44)
    • Extended API for easier CSS property comparison (#45)
  • v5.0.4 - 2017-04-06
    • Improved special identifier handling in expressions (#38)
    • Added support for css-grid spec with new "line-name" syntax element
    • Added new CSS units: fr, vmax and Q
  • v5.0.3 - 2017-01-09
    • Binds to ph-commons 8.6.0
    • Added custom callback for illegal characters
  • v5.0.2 - 2016-10-21
    • Made tab size configurable (issue #29)
    • Improved media expressions (issue #30)
    • Allowing to disable consistency checks in class CSSValue
    • Made CSS interpretation warnings customizable with class ICSSInterpretErrorHandler (issue #33)
  • v5.0.1 - 2016-08-17
    • Using "modern java template" for JavaCC parser - results in quicker execution
    • Enhancement issue #27
    • Integrated ph-csscompress-maven-plugin into this repository
    • Bug fix wrong OutputStream (issue #28)
  • v5.0.0 - 2016-06-12
    • Using JDK8 as the basis
    • removed explicit grammar for CSS 2.1 (issue #20)
    • Added browser compliant error handler
  • v4.2.0 - 2018-03-22
    • Removed dependency upon java.awt.Color for improved Android compatibility.
  • v4.1.6 - 2016-10-21
    • Made CSS interpretation warnings customizable with class ICSSInterpretErrorHandler (issue #33)
  • v4.1.5 - 2016-09-16
    • Improved media expressions (issue #30)
    • Integrated ph-csscompress-maven-plugin into this project (compatibility to 5.x)
  • v4.1.4 - Improved browser compliant parsing (issue #26)
  • v4.1.3 - Improved browser compliant parsing (issue #24)
  • v4.1.2 - Improved browser compliant parsing (issue #21)
  • v4.1.1 - Skipping style rules with invalid selectors in browser compliant mode; allowing "--" identifier prefix; improved unknown rule parsing (issues #17, #18, #19)
  • v4.1.0 - Improved calc parsing; extended support for page rules with page margin blocks
  • v4.0.1 - Added customizable newline characters
  • v4.0.0 - Updated to ph-commons 6.0.0 and added "browser compliant parsing mode"
  • v3.9.2 - Updated to ph-commons 5.7.1 and fix for some minor issues
  • v3.9.1 - Updated to ph-commons 5.6.0 and fix for some shorthand handling
  • v3.9.0 - API improvements and support for vendor specific "-calc" added
  • v3.8.2 - small bugfix release
  • v3.8.1 - improvements for expression parsing and single line comment handling
  • v3.8.0 - initial version in com.helger group and package

My personal Coding Styleguide | On Twitter: @philiphelger | Kindly supported by YourKit Java Profiler

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