You can normally use the guideline that 1 teaspoon is equal to 5 ml and 1 tablespoon is equal to 15 ml. However, do make sure that the website is American and not Australian, as the tablespoon measurement as used in that country equals 20 ml.
There's a helpful table comparing the definition of the teaspoon and tablespoon measurements as used in different English-speaking countries on this Wikipedia page (in the section "Metric measures"):
http://en.wikipedia.org/wiki/Cooking_weights_and_measures
Though your specific question was about a modern website, others looking into this question may need to keep in mind that teaspoon and tablespoon measures can vary widely in older books published in different countries or even in the same country. I found a blog post with a list of examples for the tablespoon measure from different books varying from as little as 12.5 ml to as much as 25 ml:
- Margaret Powell (British) 1970 Tablespoon = 18 ml
- Australian and New Zealand Complete cooking 1973 = Tablespoon = 25 ml
- Best of Cooking (Hamlyn) - Tablespoon, (Australia) = 20 ml
- Best of Cooking (Hamlyn) - Tablespoon, (British) = 17,7 ml
- Best of Cooking (Hamlyn) - Tablespoon, (America) 14,2
- Indian Cooking, Chowhary 1952 - = Tablespoon = 25 ml
- Cook and Enjoy, De Villiers 1971 (South Africa) = Tablespoon = 12,5 ml
- Complete South African Cookbook (South Africa) 1979 Tablespoon = 12,5
- The Australian Women's weekly 1978 = 20 ml
(Source: http://whitegranny.blogspot.com/2008/11/beware-of-tablespoon.html)
And, though again this is probably not relevant to your specific question, it's interesting to note that in Dutch recipes "a teaspoon" refers to a 3 ml measurement, while "a coffee spoon" refers to a 5 ml measurement. (Source: http://www.dekooktips.com/wegen-meten.htm, in Dutch)