Hi Thomas,
Thank you for using Syncfusion products.
To resolve the TabStrip issue we
suggest you to set “<!DOCTYPE>” declaration of your “site.master” from
‘strict’ to ‘trasitional’.
For more information about
<!DOCTYPE> please refer the following link,
http://www.w3schools.com/tags/tag_doctype.asp
For your convenience we have prepared a sample to exhibit
this behavior and it is available in the following link,
Tabstrip_theme_check.zip
Please try this on your sample and let us know if the error
is still reproduced.
Regards
Dharmalingam S