# HTML guides This page lists the guides for using HTML. [Content categories](guides/content_categories) Most [HTML](index) elements are a member of one or more **content categories** — these categories group elements that share common characteristics. This is a loose grouping (it doesn't actually create a relationship among elements of these categories), but they help define and describe the categories' shared behavior and their associated rules. It's possible for elements (such as `track`) to not be a member of _any_ of these categories. [HTML cheatsheet for syntax and common tasks](guides/cheatsheet) While using HTML it can be very handy to have an easy way to remember how to use HTML tags properly and how to apply them. MDN provides you with extended [HTML reference documentation](reference/elements) as well as a deep instructional [set of HTML guides](https://developer.mozilla.org/en- US/docs/Learn_web_development/Core/Structuring_content). However, in many cases we just need some quick hints as we go. That's the whole purpose of the cheat sheet, to give you some quick accurate ready to use code snippets for common usages. [Understanding quirks and standards modes](guides/quirks_mode_and_standards_mode) In the old days of the web, pages were typically written in two versions: One for Netscape Navigator, and one for Microsoft Internet Explorer. When the web standards were made at W3C, browsers could not just start using them, as doing so would break most existing sites on the web. Browsers therefore introduced two modes to treat new standards compliant sites differently from old legacy sites. [Using date and time formats in HTML](guides/date_and_time_formats) Certain HTML elements use date and/or time values. The formats of the strings that specify these values are described in this article. [Using HTML comments ](guides/comments) An HTML **comment** is used to add explanatory notes to the markup or to prevent the browser from interpreting specific parts of the document. [Using HTML form validation and the Constraint Validation API](guides/constraint_validation) The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. [HTML5](https://developer.mozilla.org/en-US/docs/Glossary/HTML5) introduced new mechanisms for forms: it added new semantic types for the `input` element and _constraint validation_ to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the Constraint Validation API. [Using microdata in HTML](guides/microdata) Microdata is part of the WHATWG HTML Standard and is used to nest metadata within existing content on web pages. Search engines and web crawlers can extract and process microdata from a web page and use it to provide a richer browsing experience for users. Search engines benefit greatly from direct access to this structured data because it allows search engines to understand the information on web pages and provide more relevant results to users. Microdata uses a supporting vocabulary to describe an item and name-value pairs to assign values to its properties. Microdata is an attempt to provide a declarative way of annotating HTML elements with machine-readable tags than the similar approaches of using RDFa and classic microformats. [Using microformats in HTML](guides/microformats) [_Microformats_](https://microformats.org/wiki/Main_Page) are standards used to embed semantics and structured data in HTML, and provide an API to be used by social web applications, search engines, aggregators, and other tools. These minimal patterns of HTML are used for marking up entities that range from fundamental to domain-specific information, such as people, organizations, events, and locations. [Using responsive images in HTML](guides/responsive_images) In this article, we'll learn about the concept of responsive images — images that work well on devices with widely differing screen sizes, resolutions, and other such features — and look at what tools HTML provides to help implement them. This helps to improve performance across different devices. [Using the viewport meta element](guides/viewport_meta_element) This article describes how to use the "viewport" `` tag to control the viewport's size and shape. # HTML cheatsheet for syntax and common tasks While using [HTML](https://developer.mozilla.org/en-US/docs/Glossary/HTML) it can be very handy to have an easy way to remember how to use HTML tags properly and how to apply them. MDN provides you with extended [HTML reference documentation](../reference/elements) as well as a deep instructional [set of HTML guides](https://developer.mozilla.org/en- US/docs/Learn_web_development/Core/Structuring_content). However, in many cases we just need some quick hints as we go. That's the whole purpose of the cheat sheet, to give you some quick accurate ready to use code snippets for common usages. **Note:** HTML tags must be used for their semantic value, not their appearance. It's always possible to totally change the look and feel of a given tag using [CSS](https://developer.mozilla.org/en-US/docs/Glossary/CSS) so, when using HTML, take the time to focus on the meaning rather than the appearance. ## Inline elements An "element" is a single part of a webpage. Some elements are large and hold smaller elements like containers. Some elements are small and are "nested" inside larger ones. By default, "inline elements" appear next to one another in a webpage. They take up only as much width as they need in a page and fit together horizontally like words in a sentence or books shelved side-by-side in a row. All inline elements can be placed within the `
` element. Inline elements: usage and examples Usage | Element | Example ---|---|--- A link | [``](../reference/elements/a) | A link to example.org. An image | [``](../reference/elements/q) |Me?, she said. A line break | [`
`](../reference/elements/br) | Line 1
Line 2 A possible line break | [``](../reference/elements/wbr) | LlanfairDate | [`pwllgwyngyll gogerychwyrndrobwllllantysiliogogogoch.