In JavaScript, @supports can be accessed via the CSS object model interface CSSSupportsRule.

The @supports at-rule consists of a block of statements with a supports condition. The supports condition is a set of one or more name-value pairs (e.g., <property>: <value>).

@supports (<supports-condition>) {
  /* If the condition is true, use the CSS in this block. */
}

The conditions can be combined by conjunctions (and), disjunctions (or), and/or negations (not).

@supports (<supports-condition>) and (<supports-condition>) {
  /* If both conditions are true, use the CSS in this block. */
}

The precedence of operators can be defined with parentheses. Supports conditions can use either a <property>: <value> declaration syntax or a <function()> syntax. The following sections describe the use of each type of supports condition.

The declaration syntax checks if a browser supports the specified <property>: <value> declaration. The declaration must be surrounded by parentheses. The following example returns true and applies the CSS style if the browser supports the expression transform-origin: 5% 5%:

@supports (transform-origin: 5% 5%) {
}

The function syntax checks if a browser supports values or expressions within the function. The functions supported in the function syntax are described in the following sections.

selector() Experimental

This function evaluates if a browser supports the specified selector syntax. The following example returns true and applies the CSS style if the browser supports the child combinator:

@supports selector(h2 > p) {
}

font-tech()

This function checks if a browser supports the specified font technology for layout and rendering. The following example returns true and applies the CSS style if the browser supports the COLRv1 font technology:

@supports font-tech(color-COLRv1) {
}

The table below describes the available font technologies that can be queried using this function:

font-format()

This function checks if a browser supports the specified font format for layout and rendering. The following example returns true and applies the CSS style if the browser supports the opentype font format:

@supports font-format(opentype) {
}

The following table describes the available formats that can be queried with this function:

The not operator precedes an expression resulting in the negation of the expression. The following returns true if the browser's transform-origin property considers 10em 10em 10em to be invalid:

@supports not (transform-origin: 10em 10em 10em) {
}

As with any operator, the not operator can be applied to a declaration of any complexity. The following examples are both valid:

@supports not (not (transform-origin: 2px)) {
}
@supports (display: grid) and (not (display: inline-grid)) {
}

The and operator creates a new expression from the conjunction of two shorter expressions. It returns true only if both of the shorter expressions are also true. The following example returns true if and only if the two shorter expressions are simultaneously true:

@supports (display: table-cell) and (display: list-item) {
}

Multiple conjunctions can be juxtaposed without the need of more parentheses. The following are both equivalent:

@supports (display: table-cell) and (display: list-item) and (display: contents) {
}
@supports (display: table-cell) and
  ((display: list-item) and (display: contents)) {
}

The or operator creates a new expression from the disjunction of two shorter expressions. It returns true if one or both of the shorter expressions is also true. The following example returns true if at least one of the two shorter expressions is true:

@supports (transform-style: preserve) or (-moz-transform-style: preserve) {
}

Multiple disjunctions can be juxtaposed without the need of more parentheses. The following are both equivalent:

@supports (transform-style: preserve) or (-moz-transform-style: preserve) or (-webkit-transform-style: preserve) {}

@supports (transform-style: preserve-3d) or ((-moz-transform-style: preserve-3d) or (-webkit-transform-style: preserve-3d))) {}

@supports = 
  @supports <supports-condition> { <stylesheet> }  

<supports-condition> = 
  not <supports-in-parens>                            |
  <supports-in-parens> [ and <supports-in-parens> ]*  |
  <supports-in-parens> [ or <supports-in-parens> ]*   

<supports-in-parens> = 
  ( <supports-condition> )  |
  <supports-feature>        |
  <general-enclosed>        

<supports-feature> = 
  <supports-decl>  

<general-enclosed> = 
  [ <function-token> <any-value>? ) ]  |
  [ ( <any-value>? ) ]                 

<supports-decl> = 
  ( <declaration> )  

CSS conditional rules provide the ability to test for the support of a selector such as :has().

/* This rule won't be applied in browsers that don't support :has() */
ul:has(> li li) {
  /* CSS is applied when the :has(…) pseudo-class is supported */
}

@supports not selector(:has(a, b)) {
  /* Fallback for when :has() is unsupported */
  ul > li,
  ol > li {
    /* The above expanded for browsers that don't support :has(…) */
  }
}

/* Note: So far, there's no browser that supports the `of` argument of :nth-child(…) */
@supports selector(:nth-child(1n of a, b)) {
  /* This rule needs to be inside the @supports block, otherwise
     it will be partially applied in browsers which don't support
     the `of` argument of :nth-child(…) */
  :is(:nth-child(1n of ul, ol) a, details > summary) {
    /* CSS applied when the :is(…) selector and
       the `of` argument of :nth-child(…) are both supported */
  }
}

The following example applies the CSS style if the browser supports the COLRv1 font technology:

@import url("https://fonts.googleapis.com/css2?family=Bungee+Spice");

@supports font-tech(color-COLRv1) {
  font-family: "Bungee Spice";
}

It's also possible to test for the support of a font technology by using the tech function inside the @font-face at-rule. If a browser doesn't support the font technology, a fallback font (Bungee-fallback.otf) can be used instead.

@font-face {
  font-family: "Bungee Spice";
  src: url("https://fonts.googleapis.com/css2?family=Bungee+Spice") tech(color-COLRv1),
    url("Bungee-fallback.otf") format("opentype");
}

The following example applies the CSS style if the browser supports the woff2 font format:

@supports font-format(woff2) {
  font-family: "Open Sans";
  src: url("open-sans.woff2") format("woff2");
}

• Using feature queries
• The CSSOM class CSSSupportsRule, and the CSS.supports() method that allows the same check to be performed via JavaScript.