Is your site XHMTL compliant? Don’t know? “Run it through” the “Validator” at w.3.org.
It’s not uncommon for a site to contain many errors. Three reasons you may get errors are:
1) Your theme contains errors
2) Your plugins or widgets contain errors
3) Your advertising code contains errors.
To find out if a new theme you’ve uploaded contains errors, run it through the Markup Validation Service, before you add any plugins or posts to your site. If you find errors in a theme you’ve just uploaded, and you’re comfortable with HTML, try fixing them. If not, you may be better off, finding a theme which is compliant.
As you’re adding your plugins, test them one by one. If a plugin creates an error, you may decide not to use it.
As you add your advertising HTML code, check to see if it’s compliant. If it’s not, try a different ad format, or don’t use the ads from that particular advertiser.
If you correct the HTML code, remember to retest your site.
Remember to always keep a backup of your pages.
In the event you create problems on your site that deems it inoperable (and you didn’t keep a backup), go to your FTP client file, where you can “re-upload” the respective pages. If you’re not sure where you’ve messed up, delete the respective theme, and upload it again. You’ll have lost any changes you’ve made, however, your site should be back up and running.
If your site has a lot of posts, save a copy to your hard drive, before you start trying to correct HTML code. If you don’t save a backup, you could risk losing all of the hours you’ve invested in your blog development, plus your ratings and traffic counts.
BTW: When I first ran this site through the Validator, I had over 90 errors. I disabled several of my plugins and now am down to 23 errors.