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There are two main varieties of HTML editors: text and WYSIWYG (what you see is what you get) editors.
Text editors intended for use with HTML usually provide at least syntax highlighting. Some editors additionally feature templates, toolbars and keyboard shortcuts to quickly insert common HTML elements and structures. Wizards, tooltip prompts and autocompletion may help with common tasks.
Text editors commonly used for HTML typically include either built-in functions or integration with external tools for such tasks as version control, link-checking and validation, code cleanup and formatting, spell-checking, uploading by FTP or WebDAV, and structuring as a project. Some functions, such as link checking or validation may use online tools, requiring a network connection.
To ease this requirement, some editors allow editing of the markup in more visually organized modes than simple color highlighting, but in modes not considered WYSIWYG. These editors typically include the option of using palette windows or dialog boxes to edit the text-based parameters of selected objects. These palettes allow editing parameters in individual fields, or inserting new tags by filling out an onscreen form, and may include additional widgets to present and select options when editing parameters (such as previewing an image or text styles) or an outline editor to expand and collapse HTML objects and properties.
WYSIWYG HTML editors provide an editing interface which resembles how the page will be displayed in a web browser. Because using a WYSIWYG editor may not require any HTML knowledge, they are often easier for an inexperienced computer user to get started with.
The WYSIWYG view is achieved by embedding a layout engine. This may be custom-written or based upon one used in a web browser. The goal is that, at all times during editing, the rendered result should represent what will be seen later in a typical web browser.
WYSIWYM (what you see is what you mean) is an alternative paradigm to WYSIWYG editors. Instead of focusing on the format or presentation of the document, it preserves the intended meaning of each element. For example, page headers, sections, paragraphs, etc. are labeled as such in the editing program, and displayed appropriately in the browser.
A given HTML document will have an inconsistent appearance on various platforms and computers for several reasons:
HTML is a structured markup language. There are certain rules on how HTML must be written if it is to conform to W3C standards for the World Wide Web. Following these rules means that web sites are accessible on all types and makes of computer, to able-bodied and people with disabilities, and also on wireless devices like mobile phones and PDAs, with their limited bandwidths and screen sizes. However, most HTML documents on the web do not meet the requirements of W3C standards. In a study conducted in 2011 on the 350 most popular web sites (selected by the Alexa index), 94 percent of websites fail the web standards markup and style sheet validation tests, or apply character encoding improperly. Even those syntactically correct documents may be inefficient due to an unnecessary use of repetition, or based upon rules that have been deprecated for some years. Current W3C recommendations on the use of CSS with HTML were first formalised by W3C in 1996 and have been revised and refined since then.
These guidelines emphasise the separation of content (HTML or XHTML) from style (CSS). This has the benefit of delivering the style information once for a whole site, not repeated in each page, let alone in each HTML element. WYSIWYG editor designers have been struggling ever since with how best to present these concepts to their users without confusing them by exposing the underlying reality. Modern WYSIWYG editors all succeed in this to some extent, but none of them has succeeded entirely.
However a web page was created or edited, WYSIWYG or by hand, in order to be successful among the greatest possible number of readers and viewers, as well as to maintain the 'worldwide' value of the Web itself, first and foremost it should consist of valid markup and code. It should not be considered ready for the World Wide Web, until its HTML and CSS syntax have been successfully validated using either the free W3C validator services (W3C HTML Validator and W3C CSS Validator) or some other trustworthy alternatives.
Accessibility of web pages by those with physical, eyesight or other disabilities is not only a good idea considering the ubiquity and importance of the web in modern society, but is also mandated by law. In the U.S., the Americans with Disabilities Act and in the UK, the Disability Discrimination Act place requirement on web sites operated by publicly funded organizations. In many other countries similar laws either already exist or soon will. Making pages accessible is more complex than just making them valid; that is a prerequisite but there are many other factors to be considered. Good web design, whether done using a WYSIWYG tool or not needs to take account of these too.
Whatever software tools are used to design, create and maintain web pages, the quality of the underlying HTML is dependent on the skill of the person who works on the page. Some knowledge of HTML, CSS and other scripting languages as well as a familiarity with the current W3C recommendations in these areas will help any designer produce better web pages, with a WYSIWYG HTML editor and without.