<center> computes to text-align: -webkit-center, which tables reset

Fact

The HTML <center> element is not text-align: center. Browsers give it text-align: -webkit-center (-moz-center in Gecko), a value that centres inline content and centres block-level children that have a width — which is how a page wrapped in <center> gets its whole layout table centred without any margin: 0 auto.

That value inherits into ordinary descendants but stops at a table. Measured in Chromium, inside a <center>: a <p>, <div> and <span> all compute to -webkit-center, while the <table> computes to start, and its cells inherit start from it.

Why it matters

It is two rules, and implementing either half alone is visibly wrong. Treat <center> as plain text-align: center and the old table-based layout it wraps is no longer centred on the page. Let the centring inherit the way a normal value would, and every cell of every table inside it comes out centred — which is how a site whose text is left-aligned in a real browser renders as a column of centred text instead.

How to apply

• In a user-agent stylesheet, write both halves:

  center { text-align: -webkit-center }
  table  { text-align: start }

• Treat -webkit-center and -moz-center as center when aligning inline content, and separately centre block children whose used width is smaller than the containing block.
• The same pair of rules is why <table align="center"> still has to work: a presentational attribute outranks the user-agent stylesheet, so the table { text-align: start } rule must not be able to override it.