HTML and XHTML exist today in two flavors: strict and transitional. By default, a lot of editing tools or CMS use the transitional model.
This is a shame because it may lead people to use a deprecated HTML syntax for presenting content instead of using CSS. This will lead to future incompatibilities and extensive use of the "Quirks mode".
I will summarize below the main differences between the strict and transitional models. Then, the reader may be able to figure out whether he can safely switch to (X)HTML strict or not.
Elements only allowed in transitional (X)HTML
The list below has been compiled using this full XHTML doctypes element comparison
Today, only the iframe element is still used widespread, despite the fact that it can be replaced by
Attributes only allowed in transitional (X)HTML
The list below has been compiled using this full XHTML doctypes element attributes comparison
alignin anything but tables-related elements,
As you can see, most of these attributes can easily be replaced by CSS rules. The only one that is still used widespread is
Document structure constraints in strict (X)HTMLThe
formmust only contain block-level elements like
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