Highlight activecell's row and column

Sure, but with one caveat: this will wipe out all conditional formatting instead.

This code uses the same logic, but instead applie temporary Conditional Formatting to the row and column which has the benefit of not overwriting normal cell formatting.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

Dim rowNumberValue As Integer, columnNumberValue As Integer, i As Integer, j As Integer

Cells.FormatConditions.Delete

rowNumberValue = ActiveCell.Row
columnNumberValue = ActiveCell.Column

For i = 1 To rowNumberValue - 1
    With Cells(i, columnNumberValue)
        .FormatConditions.Add Type:=xlExpression, Formula1:="=(1=1)"
        .FormatConditions(1).Interior.Color = 5296274
    End With
Next i

For j = 1 To columnNumberValue - 1

    With Cells(rowNumberValue, j)
        .FormatConditions.Add Type:=xlExpression, Formula1:="=(1=1)"
        .FormatConditions(1).Interior.Color = 5296274
    End With
Next j
    
End Sub

I went a similar route.

A Sheet specific macro enters the activecell address into p1

That triggers two conditional formats.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Range("P1").Value = Target.Address
End Sub

Formula:

=COLUMN()=COLUMN(INDIRECT($P$1))

Formula:

=ROW()=ROW(INDIRECT($P$1))

I think so,

Just and an and to the conditional formats

Formula:

=AND(COLUMN()=COLUMN(INDIRECT($P$1)),ROW()<=ROW(INDIRECT($P$1)))

=AND(ROW()=ROW(INDIRECT($P$1)),COLUMN()<=COLUMN(INDIRECT($P$1)))