Reference category
Web & HTTP
HTTP is the layer where most client–server failures become visible. A status code says what the server decided, headers say how the message must be handled, and a media type says how the body must be read. These pages cover all three together because real debugging rarely needs only one of them.
Start with the status code when a request fails, then read the headers that control caching, authentication and body parsing. Every entry states the standard definition first, then the causes and fixes seen in logs, so the page is useful before you open the specification.
Reference sets in this category (3)
HTTP Status Codes — Meanings, Causes and Fixes
What each HTTP status code means, when a server should return it, what causes it in practice and how to fix it. Facts from RFC 9110, paired with free debugging tools.
30 entries →HTTP Header Reference
What the common HTTP request and response headers mean, how they are parsed, and which ones carry security consequences. Facts from RFC 9110, RFC 9111 and RFC 6265, paired with a header inspector.
15 entries →MIME Types and Content Types — Reference
What each common media type is for, which file extensions map to it, how browsers and servers treat it, and the mistakes that break uploads and APIs.
23 entries →All 68 entries
HTTP Status Codes — Meanings, Causes and Fixes 30 entries
100 Continue100 Continue is an interim response that tells the client the request headers were accepted and the body can be sent.
101 Switching Protocols101 Switching Protocols confirms that the server is switching to a protocol requested in the Upgrade header.
200 OK200 OK means the request succeeded and the response body contains the representation the client asked for.
201 Created201 Created means the request succeeded and a new resource was created, usually identified by a Location header.
204 No Content204 No Content means the request succeeded but there is no body to return.
206 Partial Content206 Partial Content returns one requested byte range of a resource instead of the whole representation.
301 Moved Permanently301 Moved Permanently says the resource now lives at a new URL and future requests should use that URL.
302 Found302 Found says the resource is temporarily available at another URL, and the original URL should remain the address clients use.
303 See Other303 tells the client to fetch another URL with GET, usually after a form submission.
304 Not Modified304 Not Modified tells a client that its cached copy is still fresh, so no body needs to be transferred.
307 Temporary Redirect307 Temporary Redirect moves a request to another URL while preserving its method and body.
308 Permanent Redirect308 Permanent Redirect is a permanent move that preserves the request method and body.
400 Bad Request400 Bad Request means the server cannot process the request because the client sent something malformed.
401 Unauthorized401 Unauthorized means the request lacks valid authentication credentials for the target resource.
403 Forbidden403 Forbidden means the server understood the request but refuses to authorize it.
404 Not Found404 Not Found means the server cannot find a current representation for the target URL.
405 Method Not Allowed405 Method Not Allowed means the URL exists but does not support the HTTP method used in the request.
409 Conflict409 Conflict means the request conflicts with the current state of the target resource.
410 Gone410 Gone means the target resource is permanently unavailable and the server has no forwarding address.
412 Precondition FailedA conditional request failed because the resource no longer matches the client’s validator.
413 Content Too LargeThe request body exceeds the limit the server is willing to process.
416 Range Not SatisfiableA Range request asks for bytes outside the resource, or the server cannot satisfy the range.
422 Unprocessable Content422 Unprocessable Content means the request was well formed but its instructions could not be processed.
426 Upgrade RequiredThe server refuses the request until the client switches to a required protocol.
429 Too Many Requests429 Too Many Requests means the client has sent too many requests in a given amount of time.
500 Internal Server Error500 Internal Server Error means the server hit an unexpected condition and could not complete the request.
501 Not ImplementedThe server does not recognize or support the request method.
502 Bad Gateway502 Bad Gateway means a gateway or proxy received an invalid response from the upstream server.
503 Service Unavailable503 Service Unavailable means the server is temporarily unable to handle the request, often due to overload or maintenance.
504 Gateway Timeout504 Gateway Timeout means a gateway or proxy did not receive a timely response from an upstream server.
HTTP Header Reference 15 entries
HostHost names the site the client wants to reach; it is what makes virtual hosting possible.
Content-TypeContent-Type states the media type and optional parameters of the request or response body.
Content-LengthContent-Length is the exact body size in bytes, used to delimit messages and detect truncation.
Cache-ControlCache-Control tells browsers and shared caches whether, and for how long, they may store a response.
ETagETag identifies a specific version of a representation so caches and clients can revalidate it cheaply.
AuthorizationAuthorization carries credentials that prove the client may access the resource.
WWW-AuthenticateWWW-Authenticate describes the authentication scheme a 401 response expects.
CookieCookie sends stored name-value pairs from the client back to the server.
Set-CookieSet-Cookie asks the client to store a cookie and defines its scope, lifetime and security attributes.
AcceptAccept lists the media types the client prefers for the response.
Accept-EncodingAccept-Encoding lists the compression formats the client can decode.
User-AgentUser-Agent identifies the client software that made the request.
RefererReferer tells the server which page linked to the requested resource.
Content-Security-PolicyContent-Security-Policy restricts which sources a page may load and execute.
Strict-Transport-SecurityStrict-Transport-Security tells browsers to use HTTPS for this host for a stated period.
MIME Types and Content Types — Reference 23 entries
application/gzipThe media type for gzip-compressed files (.gz), distinct from Content-Encoding: gzip.
application/jsonapplication/json is the media type for JSON documents exchanged between clients and servers.
application/ld+jsonapplication/ld+json is JSON-LD: JSON that encodes linked-data structured markup.
application/octet-streamapplication/octet-stream is the generic media type for arbitrary binary data with no more specific type.
application/pdfapplication/pdf is the media type for PDF documents.
application/x-www-form-urlencodedapplication/x-www-form-urlencoded is the default encoding for HTML form submissions.
application/xmlapplication/xml is the media type for generic XML documents.
application/zipapplication/zip is the media type for ZIP archives.
audio/mpegaudio/mpeg is the media type for MP3 audio.
audio/oggOgg container audio, typically Vorbis or Opus, identified with a codecs parameter.
font/woff2font/woff2 is the media type for WOFF2 web fonts.
image/avifAVIF images built on the AV1 codec, with alpha and HDR support.
image/jpegimage/jpeg is the media type for JPEG photographs and other lossy raster images.
image/pngimage/png is the media type for PNG images — lossless raster graphics with transparency.
image/svg+xmlimage/svg+xml is the media type for SVG: scalable vector graphics written in XML.
image/webpimage/webp is a modern image format with lossy and lossless modes and transparency.
multipart/form-datamultipart/form-data is the media type for forms that include files or binary fields.
text/csstext/css is the media type for Cascading Style Sheets.
text/csvtext/csv is comma-separated values — a plain-text table with one record per line.
text/htmltext/html is the media type for HTML documents — the format browsers parse into a page.
text/javascripttext/javascript is the standard media type for JavaScript source files.
text/plaintext/plain is unformatted text: no markup, no structure, just characters and line breaks.
video/mp4video/mp4 is the media type for MP4 video files.