Problem with background colour of text passages

I would like to mark certain text passages with yellow colour. For this I have:
– In Customize
– Under Additional CSS
– placed the command
y {
background-color: yellow
}

– If I now write
<y>text passage</y>
then the text passage has a yellow background after the save and calling the page.
– When I call up the page again, all the marks are deleted.
– What is the reason for this?

Thanx in advance for any help…

The page I need help with: [log in to see the link]