10. A non-transitory computer-readable storage medium having computer-executable instructions stored thereon that, when executed by a computing device, cause the computing device to at least: receive a structured document; parse structural elements from the structured document; generate a text string representing a structure of the structured document from the structural elements, wherein the text string does not comprise textual content from the structured document; derive metadata from the structured document; add the metadata to the text string, the metadata representing a complexity value of the structure of the structured document based at least in part upon a maximum depth of nested structural elements in the structured document; group structural elements in the text string into N-grams utilizing a sliding window; utilize a classifier trained from a plurality of training documents labeled as belonging to a first document class to determine a probability that the structured document belongs to the first document class based on the N-grams, the classifier using the complexity value of the structure of the structured document as a coefficient for a first probability that the structured document belongs to the first document class, wherein the complexity value has a relatively lower value for a first set of documents that are similar and include a simple structure while the complexity value has a relatively higher value for a second set of documents that are relatively less similar and include a relatively more complex structure as compared to the first set of documents; determine whether the first probability that the structured document belongs to the first document class satisfies a threshold value; if the first probability that the structured document belongs to the first document class satisfies the threshold value, classifying the structured document as belonging to the first document class; and if the first probability that the structured document belongs to the first document class does not satisfy the threshold value, utilizing the classifier trained from a plurality of training documents labeled as belonging to the second document class to determine a second probability that the structured document belongs to the second document class based on the text string representing the structure of the structured document.
10. A non-transitory computer-readable storage medium having computer-executable instructions stored thereon that, when executed by a computing device, cause the computing device to at least: receive a structured document; parse structural elements from the structured document; generate a text string representing a structure of the structured document from the structural elements, wherein the text string does not comprise textual content from the structured document; derive metadata from the structured document; add the metadata to the text string, the metadata representing a complexity value of the structure of the structured document based at least in part upon a maximum depth of nested structural elements in the structured document; group structural elements in the text string into N-grams utilizing a sliding window; utilize a classifier trained from a plurality of training documents labeled as belonging to a first document class to determine a probability that the structured document belongs to the first document class based on the N-grams, the classifier using the complexity value of the structure of the structured document as a coefficient for a first probability that the structured document belongs to the first document class, wherein the complexity value has a relatively lower value for a first set of documents that are similar and include a simple structure while the complexity value has a relatively higher value for a second set of documents that are relatively less similar and include a relatively more complex structure as compared to the first set of documents; determine whether the first probability that the structured document belongs to the first document class satisfies a threshold value; if the first probability that the structured document belongs to the first document class satisfies the threshold value, classifying the structured document as belonging to the first document class; and if the first probability that the structured document belongs to the first document class does not satisfy the threshold value, utilizing the classifier trained from a plurality of training documents labeled as belonging to the second document class to determine a second probability that the structured document belongs to the second document class based on the text string representing the structure of the structured document. 13. The non-transitory computer-readable storage medium of claim 10 , wherein the structured document comprises a hypertext markup language (HTML) document and wherein the text string representing the structure of the structured document comprises a word for each of the HTML tags in the HTML document.