CSS basics - HTML5 and CSS3 - page 486

There's only one standard, so the way different browsers implement
Standards mode has (sometimes slowly) converged on the correct
implementation. But there are as many different ways to implement
Quirks mode as there are old versions of browsers, so pages that ren-
der in Quirks mode can vary wildly even among modern browsers.
In order to decide whether to use Quirks mode or Standards mode, the
browser takes various hints from the HTML . This also varies from
browser to browser; but, broadly, documents that follow the stan-
dards—which have correct markup according to the HTML4 or
XHTML1 specs—use Standards mode. Pages that don't follow the
standards, or claim to be following earlier versions of HTML , use
Quirks mode.
Generally this has worked pretty well. Web authors creating new
pages and paying attention to their markup have their pages rendered
as they expect, and old pages or pages created by unskilled authors are
rendered as expected (by users who have the same browser as the
designer, at least).
The situation has added complexity to the task of improving from an
unskilled to a skilled web author. Some errors in your markup won't
trigger Quirks mode, but other errors of apparently similar complexity
will. Authors may make small changes in their code only to have unex-
pectedly large changes in the end results. This inconsistency can be
frustrating and is one of the main reasons cross-browser web authoring
has gained a reputation for being confusing and capricious.
Right now, you need to understand that if you're seeing markedly dif-
ferent results for the same page across several modern browsers, it's
likely you've accidentally triggered Quirks mode through an error in
markup. The solution is usually to run your markup through one of the
online validators and correct any errors reported.
Display modes: inline, block, and none
Appendix B discussed the difference between block and inline ele-
ments. In addition to being a way to categorize HTML elements, these
also assume a default visual presentation. Inline elements sit in the flow