Doctype Explains: Technical Basis, Browser Behaviour and Relevance
What the Doctype does, why it is technically important and what indirect effects on rendering and quality can arise.
The topic often only becomes relevant when technical details suddenly deteriorate rendering, delivery or display quality. Clean standards save a lot of effort here in the long term.
What the Doctype does, why it is technically important and what indirect effects on rendering and quality can arise. This wiki page arranges the topic practically and shows, When priority arises, which mistakes are often and how you derive concrete measures from them.
Example: The page runs in Quirks Mode
An older template starts without a suitable doctype. In the browser, document.compatMode shows the value BackCompat. Check the actual HTML response delivered and correct the document start to a suitable HTML doctype before concealing individual layout deviations with additional CSS.
After that, test tables, distances, and forms. A valid doctype is a technical basis for predictable presentation, but not an independent promise of better rankings. It replaces neither valid side structure nor accessible controls.
What doctypes are there?
There are a number of different doctypes that are suitable for websites. Many of these are no longer up-to-date and have been replaced by the current standard Doctype HTML5, which enables semantic structuring and is especially important for mobile devices.
HTML5
HTML5 is a doctype characterized by many semantic tags, which makes it particularly popular with search engines. For example, the header, the footer, the main content area and the sidebar can be marked by their own HTML tags, which helps the search engines to interpret content better. HTML5 is now the standard for displaying web pages on desktop computers and mobile devices. HTML5 is now in the version HTML 5.2 available, the specification of which was published by the W3C on 14 December 2017.
<!DOCTYPE html>
<html>
<head>
<title>...</title>
</head>
<body>
<header>...</header>
<main>...</main>
<footer>...</footer>
</body>
</html> XHTML 1.1 Strict
XHTML 1.1 was the further development of XHMTL Transitional and connected HTML with XML structures in particular. XHTML 1.1 Strict included a more defined set of rules compared to HTML 4.01 and called for stricter syntax, resulting in improved websites with fewer errors. XHTML 1.1 Strict is considered deprecated and is no longer used these days. The XHTML 1.1 Strict Doctype has been replaced by the newer Doctype HTML5.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>...</title>
</head>
<body>
...
</body>
</html>HTML 4.01
HTML 4.01 has long been considered the standard for HTML website development. However, the more demands on the Internet were replaced by Doctypes with extended functionality and better syntax, such as XHTML 1.1 and the new Doctype HTML5. HTML 4.01 Doctype awards are outdated and should no longer be used.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>...</title>
</head>
<body>
<h1>...</h1>
</body>
</html>
Seitenreport and Doctypes
In the analysis, Seitenreport recommends the use of the HTML5 doctype for websites. If the analyzed website does not have an HTML5 Doctype, Seitenreport recommends that you update the Doctype accordingly and adjust the HTML semantics. This makes it easier, among other things, for search engines to index the content semantically correctly. HTML5 doctypes are best rated by the Seitenreport SEO and website analysis.