The stylistic errors are those violating the laws of use of correct words and word combinations in language, in general or in a given literary genre.
This application is the nearest in its tasks to normative grammars and manuals on stylistics in the printed, oriented to humans, form. Thus, style checkers play a didactic and prescriptive role for authors of texts.
For example, you are not recommended to use any vulgar words or purely colloquial constructions in official documents. As to more formal properties of Spanish texts, their sentences should not normally contain ten prepositions de, and should not be longer than, let us say, twenty lines. With respect to Spanish lexicon, it is not recommended to use the English words parking and lobby instead of estacionamiento and vestíbulo, or to use the Americanism salvar in the meaning ‘to save in memory’ instead of guardar.
In the Spanish sentence La recolección de datos en tiempo real es realizada mediante un servidor, the words in boldface contain two stylistic anomalies: se realiza is usually better than es realizada, and such a close neighborhood of words with the same stem, like real and realizada, is unwanted.
In the Spanish sentence La grabación, reproducción y simulación de datos son funciones en todos los sistemas de manipulación de información, the frequency of words with the suffix -ción oversteps limits of a good style.
The style checker should use a dictionary of words supplied with their usage marks, synonyms, information on proper use of prepositions, compatibility with other words, etc. It should also use automatic parsing, which can detect improper syntactic constructions.
There exist style checkers for English and some other major languages, but mainly in laboratory versions. Meanwhile commercial style checkers are usually rather primitive in their functions.
As a very primitive way to assess stylistic properties of a text, some commercial style checkers calculate the average length of words in the text, i.e., the number of letters in them; length of sentences, i.e., the number of words in them; length of paragraphs, i.e., the number of words and sentences. They can also use other statistical characteristics that can be easily calculated as a combination of those mentioned.
The larger the average length of a word, sentence or paragraph, the more difficult the text is to read, according to those simplest stylistic assessments. It is easy also to count the occurrences of prepositions de or nouns ending in -ción in Spanish sentences.
Such style checkers can only tell the user that the text is too complicated (awkward) for the chosen genre, but usually cannot give any specific suggestions as to how to improve the text.
The assessment of deeper and more interesting stylistic properties, connected with the lexicon and the syntactic constructions, is still considered a task for the future.