When you are writing for readers of web content then the entire content should be written at a general audience reading level. If it is not then there is a chance that your message will not be understood and the reader will leave your page.
There are some very good online tools that can be used to check the reading level of content – readability-score.com is an easy to use online tool that provides an instant result based on a grade level.
A grade level (based on the USA education system) is equivalent to the number of years of education a person has had. A score of around 10-12 is roughly the reading level on completion of high school. Text to be read by the general public should aim for a grade level of around 8.
The readability-score tool goes on to provide other text statistics as well.
But please spend the $10 to buy the tool and help support the developers of this open source resource.