Hi, RTE is not a input element but a div, you can implement custom solution like
https://stackoverflow.com/questions/20726174/placeholder-for-contenteditable-div
form.cq-dialog .cq-dialog-content .cq-RichText-editable[contenteditable=true]:before{
content: attr(placeholder);
font-size: 14px;
color: #254e72;
font-style: italic;
}
Arun Patidar