markup validator on yahoo.com
About to release a web site to production and wanted to run this in http://validator.w3.org/. I get a bunch of errors, something like "reference not terminated by REFC delimiter", "reference to external entity in attribute value", etc etc, and some legitimate "required attribute alt is not specified" etc.

I wanted to see how other sites are doing, so I tried yahoo.com and I see twice the amount of errors on it.

What errors need to be corrected or should I even bother? Perhaps this is not really very useful?
markup validator on yahoo.com
===

About to release a web site to production and wanted to run this in http://validator.w3.org/. I get a bunch of errors, something like "reference not terminated by REFC delimiter", "reference to external entity in attribute value", etc etc, and some legitimate "required attribute alt is not specified" etc.

I wanted to see how other sites are doing, so I tried yahoo.com and I see twice the amount of errors on it.

What errors need to be corrected or should I even bother? Perhaps this is not really very useful?