There are basically two ways of processing all headers/footers in a document. The following two subs demonstrate them.

The first sub loops through all sections, then through all headers/footers in the section, testing whether they're linked to a previous one.

The second sub achieves the same result more simply and more efficiently, without the need to check the LinkToPrevious state, by processing the corresponding story-ranges.

There are, of course, advantages to the first approach in some situations.
Sub Demo1()
Application.ScreenUpdating = False
Dim Fld As Field, Sctn As Section, HdFt As HeaderFooter
With ActiveDocument
  For Each Sctn In .Sections
    For Each HdFt In Sctn.Headers
      With HdFt
        If .LinkToPrevious = False Then
          With .Range.Find
            .ClearFormatting
            .Replacement.ClearFormatting
            .Format = False
            .Forward = True
            .Wrap = wdFindStop
            .Text = ""
            .Replacement.Text = ""
            .MatchCase = True
            .MatchAllWordForms = True
            .MatchWholeWord = False
            .MatchWildcards = False
            .Execute Replace:=wdReplaceAll
          End With
        End If
      End With
    Next
    For Each HdFt In Sctn.Footers
      With HdFt
        If .LinkToPrevious = False Then
          With .Range.Find
            .ClearFormatting
            .Replacement.ClearFormatting
            .Format = False
            .Forward = True
            .Wrap = wdFindStop
            .Text = ""
            .Replacement.Text = ""
            .MatchCase = True
            .MatchAllWordForms = True
            .MatchWholeWord = False
            .MatchWildcards = False
            .Execute Replace:=wdReplaceAll
          End With
        End If
      End With
    Next
  Next
End With
End Sub
Sub Demo2()
Dim Stry As Range
With ActiveDocument
  For Each Stry In .StoryRanges
    With Stry
      Select Case .StoryType
        Case wdEvenPagesHeaderStory, wdFirstPageHeaderStory, wdPrimaryHeaderStory
          With .Find
            .ClearFormatting
            .Replacement.ClearFormatting
            .Format = False
            .Forward = True
            .Wrap = wdFindStop
            .Text = ""
            .Replacement.Text = ""
            .MatchCase = True
            .MatchAllWordForms = True
            .MatchWholeWord = False
            .MatchWildcards = False
            .Execute Replace:=wdReplaceAll
          End With
        Case wdEvenPagesFooterStory, wdFirstPageFooterStory, wdPrimaryFooterStory
          With .Find
            .ClearFormatting
            .Replacement.ClearFormatting
            .Format = False
            .Forward = True
            .Wrap = wdFindStop
            .Text = ""
            .Replacement.Text = ""
            .MatchCase = True
            .MatchAllWordForms = True
            .MatchWholeWord = False
            .MatchWildcards = False
            .Execute Replace:=wdReplaceAll
          End With
      End Select
    End With
  Next
End With
End Sub
I too would prefer not to entangle myself in yet another attempt to (mis)use Word as an HTML editor.