Definitions beginning with X

x-default

Reserved hreflang value for the fallback page when none of the offered languages or regions matches the user. This annotation identifies a fallback for users not targeted by the other languages or regions. It does not force the browser’s language.

Reference: Google Search Central · Localized versions · Source consulted 2026-10-25

In practice: Point this variant to a suitable selector or landing page for people without a matching language.

X-Robots-Tag

HTTP header providing indexing directives to supporting crawlers, including for non-HTML documents. It does not protect access to a private file.

Reference: MDN : X-Robots-Tag

Inspect a document’s actual response and directive, distinguishing index exclusion from access control.

XHTML

Formulation of HTML under strict XML syntax rules, now less common on the general Web.

In practice: Check served type and actual document validity before using this syntax.

XML

Extensible markup language for structured documents and exchanges, including RSS feeds and sitemaps. A well-formed document follows syntax rules. Conformance to a schema and accuracy of its values require additional checks.

Reference: MDN : XML introduction · Source consulted 2026-10-18

In practice: Validate encoding, structure and special characters before exchanging a file.

XPath

Language used to select nodes and values in an XML document or compatible tree representation.

In practice: Test the expression on several documents, including missing or repeated elements.

XSS

Cross-Site Scripting: vulnerability allowing code to be injected and executed in other users’ browsers.

Reference: OWASP · Cross Site Scripting Prevention · Source consulted 2026-10-31

In practice: Test fields displaying user content in their HTML, attribute or script context.