• Skip to main content
  • Skip to search
  • Skip to select language
MDN Web Docs
  • References
    • Overview / Web Technology

      Web technology reference for developers

    • HTML

      Structure of content on the web

    • CSS

      Code used to describe document style

    • JavaScript

      General-purpose scripting language

    • HTTP

      Protocol for transmitting web resources

    • Web APIs

      Interfaces for building web applications

    • Web Extensions

      Developing extensions for web browsers

    • Web Technology

      Web technology reference for developers

  • Guides
    • Overview / MDN Learning Area

      Learn web development

    • MDN Learning Area

      Learn web development

    • HTML

      Learn to structure web content with HTML

    • CSS

      Learn to style content using CSS

    • JavaScript

      Learn to run scripts in the browser

    • Accessibility

      Learn to make the web accessible to all

  • Plus
    • Overview

      A customized MDN experience

    • AI Help (beta)

      Get real-time assistance and support

    • Updates

      All browser compatibility updates at a glance

    • Documentation

      Learn how to use MDN Plus

    • FAQ

      Frequently asked questions about MDN Plus

  • Blog
  • Play
  • AI Help Beta
  • Log in
  • Sign up for free
  1. MDN Web Docs Glossary: Definitions of Web-related terms
  2. Validator

In this article

  • See also
  1. MDN Web Docs Glossary
    1. Abstraction
    2. Accent
    3. Accessibility
    4. Accessibility tree
    5. Accessible name
    6. Adobe Flash
    7. Advance measure
    8. Ajax
    9. Algorithm
    10. Alignment container
    11. Alignment subject
    12. Alpha (alpha channel)
    13. ALPN
    14. API
    15. Apple Safari
    16. Application Context
    17. Argument
    18. ARIA
    19. ARPA
    20. ARPANET
    21. Array
    22. ASCII
    23. Asynchronous
    24. ATAG
    25. Attribute
    26. Bandwidth
    27. Base64
    28. Baseline
    29. Beacon
    30. Bézier curve
    31. BiDi
    32. BigInt
    33. Binding
    34. Bitwise flags
    35. Blink
    36. Block
    37. Block cipher mode of operation
    38. Block-level content
    39. Boolean
    40. Bounding Box
    41. Breadcrumb
    42. Brotli
    43. Browser
    44. Browsing context
    45. Buffer
    46. Cache
    47. Cacheable
    48. CalDAV
    49. Call stack
    50. Callback function
    51. Camel case
    52. Canonical order
    53. Canvas
    54. Card sorting
    55. CardDAV
    56. Caret
    57. CDN
    58. Certificate authority
    59. Certified
    60. Challenge-response authentication
    61. Character
    62. Character encoding
    63. Character set
    64. Chrome
    65. CIA
    66. Cipher
    67. Cipher suite
    68. Ciphertext
    69. Class
    70. Clickjacking
    71. Closure
    72. CMS
    73. Code point
    74. Code splitting
    75. Code unit
    76. Codec
    77. Color wheel
    78. Compile
    79. Compile time
    80. Composite operation
    81. Computer Programming
    82. Conditional
    83. Constant
    84. Constructor
    85. Continuous Media
    86. Control flow
    87. Cookie
    88. Copyleft
    89. CORS
    90. CORS-safelisted request header
    91. CORS-safelisted response header
    92. Crawler
    93. CRLF
    94. Cross Axis
    95. Cross-site scripting (XSS)
    96. CRUD
    97. Cryptanalysis
    98. Cryptographic hash function
    99. Cryptography
    100. CSP
    101. CSRF
    102. CSS
    103. CSS Object Model (CSSOM)
    104. CSS pixel
    105. CSS preprocessor
    106. Data structure
    107. Database
    108. Decryption
    109. Deep copy
    110. Delta
    111. Denial of Service
    112. Descriptor (CSS)
    113. Deserialization
    114. Developer Tools
    115. Digest
    116. Digital certificate
    117. Distributed Denial of Service
    118. DMZ
    119. DNS
    120. Doctype
    121. Document directive
    122. Document environment
    123. DOM (Document Object Model)
    124. Domain
    125. Domain name
    126. Domain sharding
    127. Dominator
    128. DoS attack
    129. DSL
    130. DTLS (Datagram Transport Layer Security)
    131. DTMF (Dual-Tone Multi-Frequency signaling)
    132. Dynamic typing
    133. ECMA
    134. ECMAScript
    135. Effective connection type
    136. Element
    137. Encapsulation
    138. Encryption
    139. Endianness
    140. Engine
    141. Entity
    142. Entity header
    143. Enumerated
    144. eTLD
    145. Event
    146. Exception
    147. EXIF
    148. Expando
    149. Fallback alignment
    150. Falsy
    151. Favicon
    152. Fetch directive
    153. Fetch metadata request header
    154. Fingerprinting
    155. Firefox OS
    156. Firewall
    157. First contentful paint
    158. First CPU idle
    159. First input delay
    160. First interactive
    161. First Meaningful Paint
    162. First paint
    163. First-class Function
    164. Flex
    165. Flex Container
    166. Flex Item
    167. Flexbox
    168. Forbidden header name
    169. Forbidden response header name
    170. Fork
    171. Fragmentainer
    172. Frame rate (FPS)
    173. FTP
    174. FTU
    175. Function
    176. Fuzz testing
    177. Gamut
    178. Garbage collection
    179. Gecko
    180. General header
    181. GIF
    182. Git
    183. Global object
    184. Global scope
    185. Global variable
    186. Glyph
    187. Google Chrome
    188. GPL
    189. GPU
    190. Graceful degradation
    191. Grid
    192. Grid Areas
    193. Grid Axis
    194. Grid Cell
    195. Grid Column
    196. Grid container
    197. Grid Lines
    198. Grid Row
    199. Grid Tracks
    200. Guard
    201. Gutters
    202. GZip compression
    203. Hash
    204. Head
    205. High-level programming language
    206. HMAC
    207. Hoisting
    208. Host
    209. Hotlink
    210. Houdini
    211. HPKP
    212. HSTS
    213. HTML
    214. HTML5
    215. HTTP
    216. HTTP header
    217. HTTP/2
    218. HTTP/3
    219. HTTPS
    220. HTTPS RR
    221. Hyperlink
    222. Hypertext
    223. I18N
    224. IANA
    225. ICANN
    226. ICE
    227. IDE
    228. Idempotent
    229. Identifier
    230. IDL
    231. IETF
    232. IIFE
    233. IMAP
    234. Immutable
    235. IndexedDB
    236. Information architecture
    237. Inheritance
    238. Ink overflow
    239. Inline-level content
    240. Input method editor
    241. Instance
    242. Internationalization
    243. Internet
    244. Interpolation
    245. Intrinsic size
    246. IP Address
    247. IPv4
    248. IPv6
    249. IRC
    250. ISO
    251. ISP
    252. ITU
    253. Jank
    254. Java
    255. JavaScript
    256. JPEG
    257. JSON
    258. Kebab case
    259. Key
    260. Keyword
    261. Largest contentful paint
    262. Latency
    263. Layout viewport
    264. Lazy load
    265. LGPL
    266. Ligature
    267. Literal
    268. Local scope
    269. Local variable
    270. Locale
    271. Localization
    272. Long task
    273. Loop
    274. Lossless compression
    275. Lossy compression
    276. LTR (Left To Right)
    277. Main Axis
    278. Main thread
    279. Markup
    280. MathML
    281. Media
    282. Metadata
    283. Method
    284. Microsoft Edge
    285. Microsoft Internet Explorer
    286. Middleware
    287. MIME
    288. MIME type
    289. Minification
    290. MitM
    291. Mixin
    292. Mobile First
    293. Modem
    294. Modularity
    295. Mozilla Firefox
    296. Mutable
    297. MVC
    298. Namespace
    299. NaN
    300. NAT
    301. Native
    302. Navigation directive
    303. Netscape Navigator
    304. Network throttling
    305. NNTP
    306. Node
    307. Node.js
    308. Non-normative
    309. Normative
    310. Null
    311. Nullish value
    312. Number
    313. Object
    314. Object reference
    315. OOP
    316. OpenGL
    317. OpenSSL
    318. Opera Browser
    319. Operand
    320. Operator
    321. Origin
    322. OTA
    323. OWASP
    324. P2P
    325. PAC
    326. Packet
    327. Page load time
    328. Page prediction
    329. Parameter
    330. Parent object
    331. Parse
    332. Parser
    333. Payload body
    334. Payload header
    335. PDF
    336. Perceived performance
    337. Percent-encoding
    338. PHP
    339. Pixel
    340. Placeholder names
    341. Plaintext
    342. Plugin
    343. PNG
    344. Polyfill
    345. Polymorphism
    346. POP3
    347. Port
    348. Prefetch
    349. Preflight request
    350. Prerender
    351. Presto
    352. Primitive
    353. Privileged
    354. Privileged code
    355. Progressive Enhancement
    356. Progressive web apps
    357. Promise
    358. Property
    359. Protocol
    360. Prototype
    361. Prototype-based programming
    362. Proxy server
    363. Pseudo-class
    364. Pseudo-element
    365. Pseudocode
    366. Public-key cryptography
    367. Python
    368. Quality values
    369. Quaternion
    370. QUIC
    371. RAIL
    372. Random Number Generator
    373. Raster image
    374. RDF
    375. Real User Monitoring (RUM)
    376. Recursion
    377. Reflow
    378. Regular expression
    379. Render-blocking
    380. Rendering engine
    381. Repaint
    382. Replay attack
    383. Repo
    384. Reporting directive
    385. Representation header
    386. Request header
    387. Resource Timing
    388. Response header
    389. Responsive web design
    390. REST
    391. RGB
    392. RIL
    393. Robots.txt
    394. Round Trip Time (RTT)
    395. Routers
    396. RSS
    397. Rsync
    398. RTCP (RTP Control Protocol)
    399. RTF
    400. RTL (Right to Left)
    401. RTP (Real-time Transport Protocol) and SRTP (Secure RTP)
    402. RTSP: Real-time streaming protocol
    403. Ruby
    404. Safe
    405. Same-origin policy
    406. SCM
    407. Scope
    408. Screen reader
    409. Script-supporting element
    410. Scroll container
    411. SCTP
    412. SDK (Software Development Kit)
    413. SDP
    414. Search engine
    415. Second-level Domain
    416. Secure Context
    417. Secure Sockets Layer (SSL)
    418. Selector (CSS)
    419. Self-Executing Anonymous Function
    420. Semantics
    421. SEO
    422. Serializable object
    423. Serialization
    424. Server
    425. Server Timing
    426. Session Hijacking
    427. SGML
    428. Shadow tree
    429. Shallow copy
    430. Shim
    431. Signature
    432. SIMD
    433. Simple header
    434. Simple response header
    435. SISD
    436. Site
    437. Site map
    438. SLD
    439. Sloppy mode
    440. Slug
    441. Smoke Test
    442. SMPTE (Society of Motion Picture and Television Engineers)
    443. SMTP
    444. Snake case
    445. Snap positions
    446. SOAP
    447. SPA (Single-page application)
    448. Specification
    449. Speculative parsing
    450. Speed index
    451. SQL
    452. SQL Injection
    453. SRI
    454. Stacking context
    455. State machine
    456. Statement
    457. Static method
    458. Static typing
    459. Sticky activation
    460. Strict mode
    461. String
    462. Stringifier
    463. STUN
    464. Style origin
    465. Stylesheet
    466. Submit button
    467. SVG
    468. SVN
    469. Symmetric-key cryptography
    470. Synchronous
    471. Syntax
    472. Syntax error
    473. Synthetic monitoring
    474. Table Grid Box
    475. Table Wrapper Box
    476. Tag
    477. TCP
    478. TCP handshake
    479. TCP slow start
    480. Telnet
    481. Texel
    482. The Khronos Group
    483. Thread
    484. Three js
    485. Time to first byte
    486. Time to interactive
    487. TLD
    488. TOFU
    489. Top layer
    490. Transient activation
    491. Transport Layer Security (TLS)
    492. Tree shaking
    493. Trident
    494. Truthy
    495. TTL
    496. TURN
    497. Type
    498. Type coercion
    499. Type conversion
    500. UDP (User Datagram Protocol)
    501. UI
    502. Undefined
    503. Unicode
    504. Unix time
    505. URI
    506. URL
    507. URN
    508. Usenet
    509. User agent
    510. UTF-8
    511. UUID
    512. UX
    513. Validator
    514. Value
    515. Variable
    516. Vendor Prefix
    517. Viewport
    518. Visual Viewport
    519. Void element
    520. VoIP
    521. W3C
    522. WAI
    523. WCAG
    524. Web performance
    525. Web server
    526. Web standards
    527. WebAssembly
    528. WebDAV
    529. WebExtensions
    530. WebGL
    531. WebIDL
    532. WebKit
    533. WebM
    534. WebP
    535. WebRTC
    536. WebSockets
    537. WebVTT
    538. WHATWG
    539. Whitespace
    540. WindowProxy
    541. World Wide Web
    542. Wrapper
    543. XForms Deprecated
    544. XHTML
    545. XInclude
    546. XLink
    547. XML
    548. XMLHttpRequest (XHR)
    549. XPath
    550. XQuery
    551. XSLT

In this article

  • See also

Validator

A validator is a program that checks for syntax errors in code. Validators can be created for any format or language, but in our context we speak of tools that check HTML, CSS, and XML.

See also

  • Validator on Wikipedia
  • Short list of validators

Found a content problem with this page?

  • Edit the page on GitHub.
  • Report the content issue.
  • View the source on GitHub.
Want to get more involved? Learn how to contribute.

This page was last modified on Jun 8, 2023 by MDN contributors.

MDN logo

Your blueprint for a better internet.

  • MDN on Twitter
  • MDN on GitHub
  • MDN Blog RSS Feed

MDN

  • About
  • Blog
  • Careers
  • Advertise with us

Support

  • Product help
  • Report an issue

Our communities

  • MDN Community
  • MDN Forum
  • MDN Chat

Developers

  • Web Technologies
  • Learn Web Development
  • MDN Plus
  • Hacks Blog
Mozilla logo
  • Website Privacy Notice
  • Cookies
  • Legal
  • Community Participation Guidelines

Visit Mozilla Corporation’s not-for-profit parent, the Mozilla Foundation.
Portions of this content are ©1998–2023 by individual mozilla.org contributors. Content available under a Creative Commons license.