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HTML 

              
                <table id="bigdata">
</table>
!

CSS 

              
                /*
hover a cell, fill it in red
*/
#bigdata tr > *:hover {
	background: red;
}

/* 
highlight the cells in the row before the hovered cell
*/
#bigdata tr > *:has(~ *:hover) {
	background: rgba(255,0,0,0.33);
}

/*
and now the down-the-column highlighting; it’s hacky but I couldn’t think of another way to make it work with :has()
maybe https://github.com/w3c/csswg-drafts/issues/4559 would allow for a one-selector rule? not sure
*/

#bigdata tr:has(~ tr:hover > *:nth-child( 1):hover) > *:nth-child( 1),
#bigdata tr:has(~ tr:hover > *:nth-child( 2):hover) > *:nth-child( 2),
#bigdata tr:has(~ tr:hover > *:nth-child( 3):hover) > *:nth-child( 3),
#bigdata tr:has(~ tr:hover > *:nth-child( 4):hover) > *:nth-child( 4),
#bigdata tr:has(~ tr:hover > *:nth-child( 5):hover) > *:nth-child( 5),
#bigdata tr:has(~ tr:hover > *:nth-child( 6):hover) > *:nth-child( 6),
#bigdata tr:has(~ tr:hover > *:nth-child( 7):hover) > *:nth-child( 7),
#bigdata tr:has(~ tr:hover > *:nth-child( 8):hover) > *:nth-child( 8),
#bigdata tr:has(~ tr:hover > *:nth-child( 9):hover) > *:nth-child( 9),
#bigdata tr:has(~ tr:hover > *:nth-child(10):hover) > *:nth-child(10),
#bigdata tr:has(~ tr:hover > *:nth-child(11):hover) > *:nth-child(11),
#bigdata tr:has(~ tr:hover > *:nth-child(12):hover) > *:nth-child(12),
#bigdata tr:has(~ tr:hover > *:nth-child(13):hover) > *:nth-child(13),
#bigdata tr:has(~ tr:hover > *:nth-child(14):hover) > *:nth-child(14),
#bigdata tr:has(~ tr:hover > *:nth-child(15):hover) > *:nth-child(15),
#bigdata tr:has(~ tr:hover > *:nth-child(16):hover) > *:nth-child(16),
#bigdata tr:has(~ tr:hover > *:nth-child(17):hover) > *:nth-child(17),
#bigdata tr:has(~ tr:hover > *:nth-child(18):hover) > *:nth-child(18),
#bigdata tr:has(~ tr:hover > *:nth-child(19):hover) > *:nth-child(19),
#bigdata tr:has(~ tr:hover > *:nth-child(20):hover) > *:nth-child(20),
#bigdata tr:has(~ tr:hover > *:nth-child(21):hover) > *:nth-child(21),
#bigdata tr:has(~ tr:hover > *:nth-child(22):hover) > *:nth-child(22),
#bigdata tr:has(~ tr:hover > *:nth-child(23):hover) > *:nth-child(23),
#bigdata tr:has(~ tr:hover > *:nth-child(24):hover) > *:nth-child(24),
#bigdata tr:has(~ tr:hover > *:nth-child(25):hover) > *:nth-child(25) {
	background: rgba(255,0,0,0.33);
}


/*
I thought this might somehow magically work despite the structural situation, but no dice
*/
#bigdata col td:has(td:hover) {
	font-style: italic;
}


/*
the following is other styling of the table and cells
*/

#bigdata {
	border-spacing: 0;
}
#bigdata tr:nth-child(odd) {
	background: #EEE;
}
#bigdata tr > * {
	padding-block: 0.33em;
	padding-inline: 0.5em;
	transition-duration: 150ms;
}
!

JS 

              
                let table = document.getElementById('bigdata');
let rows = 25;
let cols = 25;

let colgroup = document.createElement('colgroup');
for (i = 0; i < rows; i++) {
	let newcol = document.createElement('col');
	colgroup.appendChild(newcol);
}
table.appendChild(colgroup);

for (i = 0; i < rows; i++) {
	let newRow = document.createElement('tr');
	for (j = 0; j < cols; j++) {
		let newCell = document.createElement('td');
		newCell.textContent = Math.round(Math.random() * 10000);
		newRow.appendChild(newCell);
	}
	table.appendChild(newRow);
}
!
999px

Console