To reproduce: 
- add a RadRichTextBox and use the following code

            RadDocument document = null;
            HtmlFormatProvider  provider = new HtmlFormatProvider ();
            OpenFileDialog openDialog = new OpenFileDialog();
            openDialog.Filter = "Documents|*.html";
            openDialog.Multiselect = false;
            DialogResult dialogResult = openDialog.ShowDialog();
            if (dialogResult == System.Windows.Forms.DialogResult.OK)
            {
                using (FileStream stream = new FileStream(openDialog.FileName, FileMode.Open))
                {
                    document = provider.Import(stream);
                }
            }
            this.radRichTextBox1.Document = document;

Note: in the html file use the following way of commenting the style tag:

<style>
<!--
some style content
-->
</style>

Workaround: use the following way of commenting the content:
<!--<style>
some style content
</style>-->


Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.