CSS - Hide td in body if th has class - css

Question

I need to hide td in the body if the th in the head has the class .isSystem
Is this possible in straight CSS?
More info: the table is built dynamically. The head/columns is an array... and the tbody/rows is another array. I'm using Angular/typescript...
I tried this: th.isSystem ~ td { text-decoration: line-through; color: red; }

Answer 1

If the table is built dynamically, then the obvious way is to use col rather than th to drive this behaviour. <col> elements have special powers which enable them to affect the cells they belong to.
table {border:1px outset;}
th, td {border:1px inset;}
col.isSystem {visibility:collapse;}
<table>
<col/><col class="isSystem"/><col/><col/>
<thead>
<tr><th>One</th> <th>Two</th> <th>Three</th> <th>Four</th></tr>
</thead>
<tbody>
<tr><td>This</td> <td>This</td> <td>This</td> <td>This</td></tr>
<tr><td>is</td> <td>is</td> <td>is</td> <td>is</td></tr>
<tr><td>the</td> <td>the</td> <td>the</td> <td>the</td></tr>
<tr><td>first</td> <td>second</td><td>third</td> <td>fourth</td></tr>
<tr><td>column</td><td>column</td><td>column</td><td>column</td></tr>
</tbody>
</table>
Disclaimer: this works as advertised in Firefox, IE11 and Edge. Chrome however... sorry.

Answer 2

Bottom Line:
No, because <td> and <th> can not be siblings since they are not proper children of a <table> and even if your source markup has them that way - the browser will adjust the markup and overrule your styles.
Long explanation:
Looking at a more JS related SO question on the subject, the browser automatically will inject <thead> and <tbody> around your <th> and <tr> (subsequently <td>) elements. <thead> and <tbody> are valid child elements of <table> - <th> and <tr> are not.
As a result, finding the siblings of <th> will only return other th tags, since they technically live in a <thead> - the <td> are in a <tr> in <tbody>
Take a look at these examples:
Example 1
Codepen with straight <th> and <tr> elements
.isSystem + .row { background:red }
<table>
<th class="isSystem">Table Heading</th>
<tr class="row">
<td>Table Item</td>
</tr>
</table>
<div class="isSystem">Div Heading</div>
<div class="row">Div Item</div>
In this example, you would expect the table row to be red... The div elements in the example do this but the <tr> doesn't
Example 2
Codepen with proper <thead> and <tbody> elements
In example 2, wrapping the table with the correct thead and tbody elements, you can acheive this:
.isSystem + .rows tr { background:red; }
<table>
<thead class="isSystem"><th>Heading</th></thead>
<tbody class="rows">
<tr class="row"><td>Item</td></tr>
</tbody>
</table>
Unfortunately if your items are dynamically generated and you can not apply your classes in this way, then your only option will be using JS to target your elements as others have already mentioned. However, I would do what's possible to create proper semantic markup first.