Given<div>   <textarea></textarea></div>Why is this<div>   <textarea class=width90></textarea></div>.width90{   width:90%;}any better than this?<div>   <textarea style=width:90%;></textarea></div>edit:I updated my example better. My question should be more related to generic styles like widths, text align, etc.
Why is it bad to use inline styling in HTML for generic styles?
html;css
Looking at your updated question, Why is this<div>   <textarea class=width90></textarea></div>.width90{   width:90%;}any better than this?<div>   <textarea style=width:90%;></textarea></div>My answer is It's not any better, and neither would pass a code review with me.As noted in the comments, if you wanted to change the width from 90% to 85%, you'd still have to make changes in many places.  It would be far better to give your <textarea> tag a semantically meaningful class name such as 'InvoiceDescription', 'container', 'col-md-1', and so on.  That way when someone later on wants to both change the width AND the font/background color/whatever, they can do it in one place.