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Content-Security-Policy: style-src-elem directive

files/en-us/web/http/reference/headers/content-security-policy/style-src-elem/index.md

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The HTTP {{HTTPHeader("Content-Security-Policy")}} (CSP) style-src-elem directive specifies valid sources for stylesheet {{HTMLElement("style")}} elements and {{HTMLElement("link")}} elements with rel="stylesheet".

The directive does not set valid sources for inline style attributes; these are set using {{CSP("style-src-attr")}} (and valid sources for all styles may be set with {{CSP("style-src")}}).

<table class="properties"> <tbody> <tr> <th scope="row">CSP version</th> <td>3</td> </tr> <tr> <th scope="row">Directive type</th> <td>{{Glossary("Fetch directive")}}</td> </tr> <tr> <th scope="row">{{CSP("default-src")}} fallback</th> <td> <p> Yes. If this directive is absent, the user agent will look for the {{CSP("style-src")}} directive, and if both of them are absent, fall back to <code>default-src</code> directive. </p> </td> </tr> </tbody> </table>

Syntax

http
Content-Security-Policy: style-src-elem 'none';
Content-Security-Policy: style-src-elem <source-expression-list>;

This directive may have one of the following values:

  • 'none'
    • : No resources of this type may be loaded. The single quotes are mandatory.
  • <source-expression-list>
    • : A space-separated list of source expression values. Resources of this type may be loaded if they match any of the given source expressions. For this directive, the same source expression values are applicable as for style-src, with the exception of 'unsafe-hashes'.

style-src-elem can be used in conjunction with {{CSP("style-src")}}:

http
Content-Security-Policy: style-src <source>;
Content-Security-Policy: style-src-elem <source>;

Examples

Violation cases

Given this CSP header:

http
Content-Security-Policy: style-src-elem https://example.com/

…the following stylesheets are blocked and won't load:

html
<link href="https://not-example.com/styles/main.css" rel="stylesheet" />

<style>
  #inline-style {
    background: red;
  }
</style>

<style>
  @import "https://not-example.com/styles/print.css" print;
</style>

…as well as styles loaded using the {{HTTPHeader("Link")}} header:

http
Link: <https://not-example.com/styles/stylesheet.css>;rel=stylesheet

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

  • {{HTTPHeader("Content-Security-Policy")}}
  • {{CSP("style-src")}}
  • {{CSP("style-src-attr")}}
  • {{HTTPHeader("Link")}} header
  • {{HTMLElement("style")}}, {{HTMLElement("link")}}
  • {{cssxref("@import")}}
  • {{domxref("CSSStyleSheet.insertRule()")}}
  • {{domxref("CSSGroupingRule.insertRule()")}}
  • {{domxref("CSSStyleDeclaration.cssText")}}