For the Internet you need to write well, using short sentences and simple words. This translates to writing for the average 12 year old – grade 7 reading level. This does not mean that you dumb-down your content; just your writing style!
Ms Word can help enormously with its grammar checking tool if you prepare your posts in Word first. In addition it can evaluate the reading level of your writing.
In Ms Word go to ‘Tools, Options’ and click on the Grammar tab. Choose Grammar and Style and tick ‘Show readability statistics’.

Ms Word - Check my grammar
Click on ‘Settings’ to choose the characteristics of ‘Style’ (in older versions choose ‘Formal’). In newer versions you can tell Word how to check your style category by category. Make sure that you ask Word to look out for passive sentences – something you should never use!

Ms Word - Settings for writing style
When you have completed your piece (in Word) run ‘Check grammar and spelling’ from the menu; it will now give you the readability scores of your writing.

Flesch Reading Ease - Kincaid Grade - Too difficult!!
Flesch Reading Ease
High is easier and low is difficult.
90 – 100 is 5th grade
60 – 70 is 8 and 9th grade
0 – 30 is for college students only
Examples: Readers Digest is around 65 (good for the Internet) while Time magazine is around 52 (style not suitable for Internet)
Flesch – Kincaid Grade
This number translates directly into the grade eg 6.3 is a style for a 6th grade student
Once you are happy with the results cut and paste your checked text from Word to NOTEPAD (a small program under Accessories – this removes the Word formating that will ruin the layout of your post). From Notepad cut and paste the cleaned text into a empty post screen in WordPress.