I don't know if these are widely accepted, but the following practices in HTML and CSS, or web design in general are pissing me off.
Using font thickness as a hover effect. Not using this should be quite obvious but still I keep seeing it. Jumpy layouts suck, mmkay?
<br /> all over the place. It's okay when formatting text (poems or such), but do not use it for creating empty space in layout, that's what CSS is for!
padding makes me crazy. It's probably one of the easiest ways to get yourself totally lost in how and why some element you've set to occupy 100% of the width, takes some extra space. There's nothing you can do with it then.
Probably the most horrible naming I've seen has been on CSS classes. Naming them
witdh100 etc. is reusable, but utterly retarded if you think about it for a while. Separating the layout from the content is the purpose of CSS, now when you've got an element with
class="red floatRight" it's no different from
color="red" float="right". You should not use the elements visual properties in class naming! Common properties that tend to become parts of the class name are darkness variations ie. if in a list every other row has to have a different shade of gray, then try not to name the classes as
some-list-row-light, but instead use the rows' non-visual properties, like odd and even.
Go ahead and post your biggest annoyances in HTML and CSS implementations!