of the webmaster guideline mentioned by Google and Yahoo is to make sure that
your HTML code is valid. It means that you don't have any unclosed HTML tag.
This is most likely occur when you write your own HTML code or editing your
HTML manually from notepad.
To check this, you can use HTML validator provided by w3c to check your web
document. Type in your full path URL or you can upload your file from your
PC and then click on submit. If you did notice unclosed tags, remember to
make a correction necessarily.