The XHTML standard characterizes three Document Type Definitions (DTDs). The most usually utilized and simple one is the XHTML Transitional archive.
XHTML 1.0 report type definitions compare to three DTDs −
There are not many XHTML components and qualities, which are accessible in one DTD yet not accessible in another DTD. In this way, while composing your XHTML report, you should choose your XHTML components or qualities cautiously. Be that as it may, XHTML validator causes you to recognize substantial and invalid components and traits.
If you don't mind check XHTML Validations for more detail on this.
XHTML 1.0 Strict
On the off chance that you are wanting to utilize Cascading Style Sheet (CSS) carefully and keeping away from to compose the greater part of the XHTML characteristics, at that point it is prescribed to utilize this DTD. An archive complying with this DTD is of the best quality.
On the off chance that you need to utilize XHTML 1.0 Strict DTD, at that point you have to incorporate the accompanying line at the highest point of your XHTML record.
<!DOCTYPE html PUBLIC "-/W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional
In the event that you are intending to utilize numerous XHTML traits just as few Cascading Style Sheet properties, at that point you ought to receive this DTD and you ought to compose your XHTML archive appropriately.
On the off chance that you need to utilize XHTML 1.0 Transitional DTD, at that point you have to incorporate the accompanying line at the highest point of your XHTML report.
<!DOCTYPE html PUBLIC "-/W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Frameset
You can utilize this when you need to utilize HTML Frames to segment the program window into at least two casings.
On the off chance that you need to utilize XHTML 1.0 Frameset DTD, at that point you have to incorporate after line at the highest point of your XHTML report.
<!DOCTYPE html PUBLIC "-/W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Note − No issue what DTD you are utilizing to compose your XHTML report; on the off chance that it is a legitimate XHTML record, at that point your archive is considered as a decent quality record.