Specifications
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RFC 3986: URI Generic Syntax
The definitive specification for URI syntax
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WHATWG URL Standard
Living standard that browsers actually implement
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RFC 2141: URN Syntax
Specification for Uniform Resource Names
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RFC 7595: Guidelines for URI Scheme Registration
How new URI schemes get registered
Essential Reading
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Cool URIs don't change
Tim Berners-Lee's influential essay on URL permanence
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Architecture of the World Wide Web, Volume One
W3C's comprehensive guide to web architecture
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URL as UI
Jakob Nielsen on URLs as user interface
Tools
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MDN: URL API
Documentation for JavaScript URL parsing and manipulation
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IANA URI Schemes Registry
Official list of registered URI schemes
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Wayback Machine
Check historical versions of URLs
Persistent Identifiers
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DOI Foundation
Digital Object Identifier system for scholarly content
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ORCID
Persistent identifiers for researchers
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ARK Identifiers
Archival Resource Keys for libraries and archives