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A fragment identifier is the component introduced by # in a URL, supplying a reference within or associated with the identified resource. In https://example.org/manual.html#setup, setup is the fragment. Interpretation depends on the representation and its rules; an HTML element identifier is a familiar example, not the only possible meaning.^[ref]

The Hypertext Transfer Protocol target excludes the fragment. A browser requesting the preceding example requests the document without #setup, then can process the fragment locally. This differs from a query string, which can help identify the requested resource.^[ref]

For an illustrative comparison, manual.html#setup and manual.html#examples can select different places in one retrieved document. Changing only the fragment therefore need not imply fetching two different document bodies. Conversely, a syntactically valid fragment does not prove that a matching destination exists in the current document.

Percent-encoding represents a byte using a percent sign followed by two hexadecimal digits. For example, byte value 32 is written %20. Non-ASCII text is first encoded as bytes: in UTF-8, é becomes the two bytes hexadecimal C3 A9, represented as %C3%A9.^[ref]

A useful example distinguishes data from delimiters. The path in https://example.org/a/b has two segments; the path in https://example.org/a%2Fb retains the encoded slash within one segment in the parsed URL record. A server may apply additional decoding, but the two spellings must not be assumed interchangeable at every processing stage.

Percent-encoding is neither encryption nor a universal instruction to decode repeatedly. The string %252F decodes once to the text %2F; a second decoding changes it again. Likewise, the plus sign is not universally a space: that convention belongs to form-style query string processing, including URLSearchParams, rather than all URL components.^[ref]

A query string is the component following ? and preceding #, or the end, in a URL. It carries data whose interpretation is defined by the resource or application. The generic syntax does not require a dictionary of names and values.^[ref]

For example, https://example.org/search?q=red&tag=a&tag=b#results has query q=red&tag=a&tag=b. An application using form-style conventions can interpret repeated tag entries as a list. URLSearchParams preserves repeated entries: getAll('tag') returns both a and b. Converting that list directly into a dictionary with one value per name can lose information.

Encoding conventions also matter. Under URLSearchParams parsing, q=a+b gives the value a b, while q=a%2Bb gives a+b. That distinction comes from the specified form parser, rather than a general law that every plus in every URL means a space.^[ref]

A useful test records both the original query string and the parsing convention. A changed spelling, changed list of values and changed server response are three different observations.

A Relative URL reference identifies a target using a base URL to supply missing context. It can contain a path, a query string or a fragment identifier, without independently specifying an entire network address. Resolution combines the reference with the base according to a defined algorithm.^[ref]

Trailing slashes provide an illuminating comparison. Resolving c against https://example.org/a/b gives https://example.org/a/c; resolving it against https://example.org/a/b/ gives https://example.org/a/b/c. In the first case the last base path segment is replaced, while in the second it names a directory-like prefix.

A parent-directory step is similarly concrete: ../image.png against https://example.org/a/b/page.html gives https://example.org/a/image.png. These outcomes follow from URL processing, not from examining an actual filesystem. No network request is needed to calculate them.^[ref]

The base is therefore part of the input. Copying the same relative spelling into a document with a different base can change its target even when every character of the reference stays unchanged.

A URL scheme is the leading component before :, selecting a family of interpretation and processing rules. For example, https is the scheme of https://example.org/a. Scheme syntax begins with an ASCII letter and then permits letters, digits, plus, hyphen and period. Comparison is case-insensitive, and standard serialization uses lowercase.^[ref]

The scheme does not imply that every URL names a remote server. Compare https://example.org/a with data:text/plain,hello: both have schemes, but the second example carries data in its own spelling rather than naming an HTTPS host. A URL parser must apply the appropriate rules rather than demand a host and port for every case.

The colon also separates a scheme from a familiar relative-path interpretation. The text a:b is parsed as a URL with scheme a, whereas ./a:b can be a Relative URL reference. Adding the initial dot and slash therefore changes the syntactic role of the colon-containing text.

Scheme-specific defaults matter too: omitting port 443 when serializing an HTTPS URL reflects that scheme's default, not a rule that all numerical ports are redundant.^[ref]