show validation erros in

I have a website in ASP.NET with C#
when I compile the project I have no errors.
When I do have an error I see besides the  new error many error in the HTML code. (validation erros)
Why I don't see these errors all the time.
I gauess this is a parameter in the debugging mode but I don't know where.
for example:

Validation (XHTML 1.0 Transitional): Element 'td' is missing its closing tag.      
Validation (XHTML 1.0 Transitional): Element 'img' is missing required attribute 'alt'.      
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Oliver AmayaConnect With a Mentor EntrepeneurCommented:
Hi, HTML validation errors are shown as warnings, to see them you have to open the Error List Window after you compile your program, it may also be the case that you have to enable the warnings to be visible in this window as well.
See this

It's about the
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" > - line in the .aspx file
ANAT2403Author Commented:
The question is how to cause the compiler to show these error messages in each compilation and not only when there are other errors.
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ANAT2403Author Commented:
this is not the case.
These validation erros were not seen at all also in the warning list window untill there was a   non validation errors and only then I could see all the errors
VS validates you'r HTML based on the
DOCUMENTTYPE declared in the aspx file
try changing the
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" ""> 

Oliver AmayaEntrepeneurCommented:
Is it enable in visual studio options?
Check the Show Errors option in Tools->Options->Text Editor->Html->Validation
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