Works as expected.

One correction though. The CSS that you provided did not work. This did.

.tabbrowser-tab[label="Placeholder"] {
min-width: 1px !important;
max-width: 1px !important;
padding: 0 !important;
margin: 0 !important;
background: #808080 !important;
opacity: 0.9 !important;
}

I tried what the instructions said but I don't think it actually did anything? Maybe I'm misunderstanding, but this doesn't seem to actually do anything?