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Form URL encoding is the application/x-www-form-urlencoded format for serializing an ordered list of name/value pairs. It is used by URLSearchParams and common form-processing conventions. Repeated names are allowed; the data model need not be a dictionary with one value per name.^[ref]

For example, the three pairs ("q", "a b+c"), ("tag", "x") and ("tag", "y") serialize as q=a+b%2Bc&tag=x&tag=y.
Here the space becomes +, while the literal plus in the value becomes %2B. Parsing the result recovers a b+c and both tag values. Replacing every plus with a space before deciding which encoding convention applies would misread data from other URL contexts.

Structural separators must also be distinguished from value data. The encoded pair q=a%26b has one value, a&b, rather than two pairs. The form parser first recognizes pair boundaries and then decodes their component data. Percent-encoding inside a component is consequently not an instruction to reinterpret the resulting characters as new separators.^[ref]

For modern serialization, text is encoded as UTF-8 bytes before the relevant bytes are percent-encoded. The name/value pair ("word", "café") therefore produces word=caf%C3%A9. This connects a text character with its byte representation rather than assigning one arbitrary percent code to the whole character.^[ref]

A query string can use this format, but generic URL syntax does not require every query to be interpreted this way. State the parser and encoding when comparing an original spelling with its recovered list of values.

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.

An HTTP representation consists of data and metadata describing a selected state or form of a resource. It is the information transferred or referenced to express that resource, rather than the resource identifier itself. A resource can have more than one available representation.^[ref]

For an invented /color resource, one representation is the plain-text bytes blue with type text/plain. Another is an HTML document displaying the same color information, with type text/html. The underlying application value can agree while the bytes and media types differ. HTTP content negotiation can help select among available representations according to request information.

An HTTP response also contains control information about the exchange, such as its HTTP status code. It is not simply another name for representation data. A HEAD response, for example, can describe a selected representation without transferring its content, while a GET can transfer that content.^[ref]

Representation metadata tells a recipient how to interpret the data. A media type concerns format, character-set information concerns text encoding, and an entity tag supplies a validator for the selected representation. These properties can differ between two forms of the same resource.

A URL therefore does not promise one immutable byte sequence. A resource can change with time or offer different forms; conversely, matching application meaning does not require byte-identical representations. When comparing responses, record which representation was selected and compare the relevant data and metadata rather than only the identifier.

An HTTP safe method has essentially read-only requested semantics: the client does not ask the server to change resource state through the operation. GET and HEAD are examples. The definition concerns what the operation requests, rather than asserting that no incidental activity occurs anywhere on the server.^[ref]

For example, reading a stored color can return blue while leaving that color unchanged. The server can still append an access-log entry. A later read can return red if some separate operation changed the color in between; safe semantics do not require an unchanging response or an immutable resource.

At the intended-state level, a read-only operation can be modeled as $f(s)=s$. It follows immediately thatThis illustrates why safe methods are also idempotent. The converse fails: deletion can map both present and absent states to absent, satisfying HTTP idempotence while changing a present resource on its first application.

The chosen HTTP request method remains meaningful even when the target URL contains a query string. Naming a destructive action in a query does not redefine GET as a method for requesting that action; the standard requires such actions to be disallowed when accessed through safe methods.^[ref]

Use this term for protocol semantics. Successful transfer, access permission and the truth of returned application data are separate questions. Interpret the method with the target and application contract, rather than deciding safety from whether a response happens to contain a body.

The Hypertext Transfer Protocol is an application-level communication protocol for exchanging requests and responses about resources. In the client-server model, an HTTP request supplies an HTTP request method and target; an HTTP response supplies an HTTP status code and associated information. HTTP header fields convey metadata, and messages can carry content. These semantics are distinct from the particular wire format used by a protocol version.^[ref]

For a concrete exchange, a client can send method GET for https://example.org/manual. A successful response can carry status 200 and an HTML representation of the resource. A response with status 404 instead says that the server did not find a current representation, or is unwilling to disclose one; successful communication does not require successful retrieval.^[ref]

The request's URL identifies the target, while the response describes the outcome of this exchange. A later exchange can return different content or a different status. The protocol's stateless request semantics also do not prevent an application from maintaining state: the distinction concerns what a request must mean, not whether a user can have an ongoing session.^[ref]

A media type identifies a data format and its intended interpretation using a type/subtype name, optionally followed by parameters. It lets a recipient choose an appropriate interpretation rather than inferring everything from a file name. Media types are used in the Hypertext Transfer Protocol and other Internet protocols.^[ref]

For example, text/plain; charset=utf-8 has type text, subtype plain and a charset parameter naming UTF-8. In an HTTP header field such as Content-Type: text/plain; charset=utf-8, this describes the accompanying representation data; it is not itself that data.^[ref]

A format and a character encoding answer different questions. A media type can describe plain text or an image format, while a character encoding explains how text characters correspond to bytes. For example, the PNG format has media type image/png; it is not interpreted by treating all its bytes as a sequence of displayed text characters.

A file suffix is another convention. Two files with the same suffix need not actually contain the same format, and a response's type metadata need not be derived from the last characters of its URL. Correct interpretation requires consistent metadata and data, not merely a suggestive name.

Type/subtype tokens and parameter names have their specified case-insensitive matching rules, but parameter values follow the rules of their own definitions. Lowercasing arbitrary values is therefore not a general parsing strategy.^[ref] A useful inspection separates the name, each parameter and the actual bytes being interpreted.

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, or Uniform Resource Locator, is a structured identifier interpreted according to a URL scheme and parsing rules. On the web it can identify the resource a browser navigates to or requests. The URL parser produces a record with components such as scheme, host, port, path, query and fragment; which components are present depends on the particular URL.^[ref]

For example, https://example.org:8443/a/b?x=1#part has scheme https, host example.org, port 8443, path /a/b, query string x=1 and fragment identifier part. This hierarchical HTTPS example is useful without implying that every URL must contain a network host or an explicit port.

A Relative URL reference needs a base. Resolving ../img/p.png against https://example.org/a/b/page.html gives https://example.org/a/img/p.png. The base's directory and parent-directory step explain the result. Parsing and serialization can also normalize a spelling: https://EXAMPLE.org:443/a serializes as https://example.org/a, lowercasing the host and omitting the default HTTPS port.^[ref]

Percent-encoding represents bytes in a URL spelling, and encoded data must be distinguished from structural delimiters. A query string is not universally a dictionary of name/value pairs, even though many applications use that convention. Nor does successful parsing prove that a resource is reachable or that its content never changes. Distinguishing syntax, identifier interpretation and an actual retrieval prevents an address from being confused with the information returned on one occasion.

A URL parser is a procedure that turns input text, and sometimes a base URL, into a structured URL record or a failure. The record separates components such as scheme, host, path, query string and fragment identifier. URL serialization converts that structured result back into text; it is a different operation from retrieving the identified resource.^[ref]

For a worked example, parsing https://EXAMPLE.org:443/a and serializing the result yields https://example.org/a. The host is normalized and the default port omitted. Successful parsing did not test whether the server exists, whether access is permitted or what content would be returned.

Failure also differs from a recoverable validation issue. The WHATWG algorithms can flag some nonconforming input while still producing a record, so an implementation should specify whether it reports validation diagnostics, a usable result or both. A regular expression recognizing one tidy address format is not automatically an implementation of the full parser.

A practical reproducibility check compares parsed fields and the serialized output under a stated parsing model. The WHATWG model and generic RFC syntax serve related purposes but should not be silently treated as identical grammars.

A URL record is the structured internal representation of a URL, rather than its original text. The URL parser produces components such as scheme, host, port, path, query string and fragment identifier; URL serialization turns the record into a string. A component can be absent, and the representation of a path depends on the scheme.^[ref]

Parse https://example.org:8443/a?x=1#part. The host is example.org, the port is 8443, the path contains a, the query is x=1, and the fragment is part. The separators ? and # belong to the serialized syntax, rather than the stored query and fragment values. These fields let a program change one component without guessing its boundaries by cutting an arbitrary string.

The original spelling need not survive. Parsing https://EXAMPLE.org:443/a yields a lowercased host and no stored explicit port, because 443 is the scheme default. Therefore the record does not contain a faithful log of every character supplied by the user. If an application needs that log, it must retain the input separately.^[ref]

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]

URLSearchParams is an API for representing and manipulating an ordered list of name/value pairs, commonly obtained from a URL's query string. Repeated names are permitted. It uses form URL encoding, rather than imposing its interpretation on every conceivable query syntax.^[ref]

For new URLSearchParams('tag=a&tag=b&q=a+b'), getAll('tag') returns both a and b, while get('q') returns a b. Calling set('tag','c') replaces the repeated entries with one: toString() then returns tag=c&q=a+b. An associative array holding just one value for each name would discard the original repeated-field information.

A literal plus needs a different spelling. Appending the pair q and a+b to a URL's searchParams produces query ?q=a%2Bb. The plus in the data is represented by percent-encoding, whereas the plus in q=a+b represents a space under this parser. These examples distinguish a character in a value from a character used by the encoding convention.^[ref]

A URL's searchParams object is connected to that URL; changes update its query. Constructing a separate URLSearchParams from a query string instead gives an independent object, useful when the original spelling must remain available for comparison.^[ref]

A web browser retrieves and presents web resources while providing an environment for interaction with them. For an HTML page, this includes parsing source into a Document Object Model, applying CSS, laying out the result and rendering it. Programs such as JavaScript can then change the document or respond to user actions.^[ref]

Consider a page containing <p id="notice">Waiting</p>. Its visible word begins as Waiting. A program can obtain that element and change its text to Ready; the Document Object Model and displayed page then change without requiring the original response bytes to change. The browser maintains a live document and rendering state, rather than repeatedly displaying an immutable picture of the source.

The visible area also has several coordinate scales. The layout viewport constrains layout, while the visual viewport describes the portion being viewed. Page zoom, pinch zoom and scrolling can change those relationships. This matters when a program places a label relative to a document element or interprets a pointer position.^[ref]

A URL identifies a resource to navigate to; a web search engine helps discover resources. A browser can present search results as a page, but navigating directly to a known address does not require a search query. The retrieval service and the software interpreting its pages remain separate parts of the system.