There are two flavors of HTML editors: text and What You See Is What You Get (WYSIWYG).
Text editors intended for use with HTML usually provide syntax highlighting. Toolbars and keyboard shortcuts may quickly insert common HTML elements and structures. Wizards, tooltip prompts and auto-completion may help with common tasks.
So-called WYSIWYG HTML editors provide an editing interface which resembles how the page will be displayed in a web browser. Most WYSIWIG editors also have a mode to edit HTML directly like a text HTML editor. Because using a WYSIWYG editor does not require any HTML knowledge, they are easier for an average computer user to get started with.
The WYSIWYG view is achieved by embedding a layout engine based upon that used in a web browser. The layout engine will have been considerably enhanced by the editor's developers to allow for typing, pasting, deleting and moving the content. The goal is that, at all times during editing, the rendered result should represent what will be seen later in a typical web browser.
WYSIWIG editors are sometimes criticized because:
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 handicapped people, and also on wireless devices like mobile phones and PDAs, with their extremely limited bandwidths and screen sizes.
Unfortunately, only about 1 out of 141 HTML documents on the web is valid according to W3C standards. 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. See CSS, XHTML, W3C's current CSS recommendation and W3C's current HTML recommendation.
These guidelines emphasise the separation of content (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 many would say that none of them yet succeed totally.
People who use text editors can generally fix such problems immediately, once they become aware of them. People find it frustrating when such errors come from WYSIWYG editors.
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 it can be argued that, first and foremost, it should consist of valid markup and code. Some would argue that it should not be delivered by a designer to his or her customer, and not be considered ready for the World Wide Web, until its HTML and CSS syntax has been successfully validated using either the free W3C validator services (W3C HTML Validator and W3C CSS Validator) or some other trustworthy alternatives.
Others would argue that publishing useful information, as soon as possible, should be first and foremost.
Whatever software tools are used to design, create and maintain web pages, there is little doubt that 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.
A given HTML document will look different to different people for several reasons: (See CSS Zen Garden for an extreme example.)
What you see may be what most visitors get, but it is not guaranteed to be what everyone gets.