What is a quality website?

We have now a definition by Google of what is a quality site, it is given by a link at bottom.

In this page, are given the criteria that define a quality site for visitors prior to search engines, we will see from the visitor's point of view what are the components of a quality site.
This become even more essential since efforts to filter these sites in search results, depending on their quality.
Although the quality of a site has always been an issue it took a particular importance in April 2011 with the Panda Update, the change of the Google algorithm to eliminate poor quality pages that even goes far in penalizing an entire site if a portion of its content is considered poor.

Quality website

Domain name
Having a true domain name and not a sub-directory on a shared hosting. The semantics of the name must be in connection with the contents of the site.
Contents
Contents must be complete, coherent, in good English and original. It will so attract backlinks and thus improve the score for search engines.
To be original with respect to search engines, content must include text with keywords that does not appear in similar articles.
The text should be organized with <h2>, <h3>, etc. tags and have a title in a <h1> tag. Do not use those tags as part of layout, CSS style sheets are there for that.
Image
Add at least one original image in each important page of the site.
It should not too big for mobile phones.
Links
No broken links, do not link to pages without interest and not links towards pages whose contents are not in close connection with your page. Check for broken links from time to time.
The title attribute of the <a> tag makes it possible to display a message when the mouse stay on the link but it is also essential for search engines. In the same way the alt attribute provides information on images.
Navigation
All pages in the site must be reached, directly or not, from the home page. Ideally the home page has at least a link to each topic in the site. It may be assisted by a site map.
Visitors
1) Must often return to the site: they will do it because they will find there useful references, unique contents, an attractive presentation.
2) Must remain a long time on the site: the pages are linked between them, the contents is interesting. One can use the techniques of the novelist, arouse interest, or even create a suspense.
Title
The title of each page must differ to that of other pages.
In the results of the search engines, one must click on your page. For that one needs a title and a description which encourage to choose this page rather than one of the others, even and especially if they are placed before your link.
The title is taken by engines in the <title> tag in the <head> section of the HTML code of the page, or an <h1> tag in the body of the page.

$title="My page"; 
<title><?php echo $title?></title>
<h1><?php echo $title?></h1>
Description
Add a meta description tag that let the user visit the page, otherwise the snippet in results, is taken by search engine at the beginning of the page, or by concatenation of sentences :
 <meta name="description" content="here the description of the page" >
Style sheets
The use of CSS makes the difference between sites of quality and sites from newbies.
CMS
Use a CMS when the site is mainly intended for news. This provide lot of tools that help the reader to find the information on the site. Otherwise use a PHP template to give a unique look and feel to all your pages.
Favicon
It is the image which is displayed in the bar of URL on the left of the name of your site. See the article about how to create the favicon of your site. It can be added in 5 minutes.
Speed
The speed of the site is now an important factor. It may be improved by reducing the number of images in a page or improving compression, moving from PNG to JPG. There are also better PNG compressors, see the list of tools.
Maslow's criteria
According to Maslow, web design must respond to a hierarchy of needs, in descending order:
  1. Accessibility: The website can be easily found, and usable by all people.
  2. Stability: The website trustworthy and all page equal.
  3. Usability: It is user-friendly.
  4. Reliability: It always available without downtime.
  5. Functionality: It offers content, tools and services users value.
  6. Flexibility: It adapts to needs and wants of users.

See also What is the quality of a website for a search engine? And how is it evaluated? En this external link, Why our newspaper is dying, a summary in image of bad practices.