In terms of SEO, for client point of view, should I use XHTML 1.0 Transitional or XHTML 1.0 Strict?
Why some people still use Transitional?
Will we lose something if We use Transitional over Strict?
XHTML Strict forces you to use semantic markup, where as Transitional is a bit more flexible and still allows you to use legacy presentational elements like <font> and <center>.
People still use Transitional to maintain support older markup, but it doesn't make any difference to SEO. Strict is considered better for encouraging more accessible XHTML and better separation of content and presentation.
In terms of SEO, no difference whatsoever.
I don't think there is any impact of using one or the other : what really matters is content, not (X)HTML (Strict/Transitional).
Echo comments that shouldn't make a difference.
In my opinion its better to pick one and make sure your site validates against it.
Also note that can make ASP.NET spit out strict by the following in the web.config
<system.web>
<xhtmlConformance mode="Strict" />
</system.web>
This is a good article explaining differences: http://24ways.org/2005/transitional-vs-strict-markup
来源：https://stackoverflow.com/questions/1885906/xhtml-1-0-transitional-vs-xhtml-1-0-strict