Since, I don't think it is possible to have both placeholder and other text string at the same time(not absolutely sure though), I am thinking to show text with look and feel of placeholder by applying some css.
Since, text area accepts any input as string, I am unable to add any css to text. I have tried with
innerHtml but it does not help.
<textarea class=" abc slds-textarea" name="textareafield"></textarea>
this.template.querySelector('.abc').innerHTML = '<b>sample</b>';
I have also tried with
<div contenteditable="true"></div> and html does get applied but since look and feel of the
div is different than textarea, I cannot use this as solution.