How can columns be semantically grouped in a table? How can a column inside a group be accessed?

By using the colgroup element and defining the span attribute indicating how many columns will the group have:

<table>
    <caption>Bidi control codes injected by unicode-bidi at the start/end of display: inline boxes</caption>
    <colgroup span="1">
    </colgroup>
    <colgroup span="2">
    </colgroup>
    <colgroup span="2">
    </colgroup>
    <thead>
...

To access a single column in a colgroup, we can use the col element:

<table>
    <caption>Bidi control codes injected by unicode-bidi at the start/end of display: inline boxes</caption>
    <colgroup>
    </colgroup>
    <colgroup>
			<col class="start">
			<col class="end">
    </colgroup>
    <colgroup>
			<col class="start">
			<col class="end">
    </colgroup>
    <thead>
...

More information: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/colgroup