I played a little bit with the accessibility features in Excel and Word, as they have been improved in a recent update. I have some notes about the Online Help article, which is then loaded in a task pane in Excel, Word or PowerPoint. I am using MS Office Version 1809 Build 10827.20084 (Monthly Targeted).
In the following, three screenshots showing an online help article, when clicking on the link in the Accessibility Task Pane. For each article, there is a table, you can find when scrolling a little bit down.
My suggestion is here to remove or change the table header background color, as then the content will not be easy to read. And, if you grayscale the image, it's more harder to read.
The Online Help article for Excel e.g. in German can be found here online. https://support.office.com/de-de/article/erstellen-von-barrierefreien-excel-tabellen-6cc05fc5-1314-48b5-8eb3-683e49b3e593
There, the table header is gray and therfore much better readable.
The article (online and in the taskpane) also includes a lot of images. I know, that the content is machine translated (and all in all ok and good), but the images for the German translation include many errors - where I guess, that text have been added by placing a layer over the screenshot.