RE: Errors,warnings and website rank.

To Valid your site is always the better way of writing,
for now and in the future.

Google search on differend keys,

-    Title
-    Meta tags keywords
-    Meta tags discription
-    Linken
-    Strong 
-    Body content
-    Adres  for example www.rustdepot.com/rustremoverconcentrate.html  works better

Be sure that the frase "rust remover concentrate" will apear  exactly in all items.

By the way on the search for "rust remover concentrate" you are number 
one of 47.500, not bad at all.

It takes a will for Google sees your new version, but it will helps to Upranking your site.

D.Bosch