corrected validation errors

Error Line 68, Column 73: Bad value button for attribute type on
element a: Subtype missing.
      <a type="button" class="navbar-toggle nav-link"
href="/about.html">
Syntax of MIME type:
A media-type as defined in RFC 2616; that is, typically, a required
type, followed by a "/" character, followed by a required subtype,
optionally followed by one or more instances of a ";" character
followed by a parameter. Examples: text/css, text/css;charset=utf-8.
Error Line 72, Column 77: An img element must have an alt attribute,
except under certain conditions. For details, consult guidance on
providing text alternatives for images.
        <img src="http://www.gravatar.com/avatar/?s=35"
class="img-circle" />
✉
Error Line 104, Column 6: Bogus comment.
   <! -- sidebar.html -->
✉
Warning Line 104, Column 8: The document is not mappable to XML 1.0 due
to two consecutive hyphens in a comment.
   <! -- sidebar.html -->
✉
Warning Line 104, Column 24: The document is not mappable to XML 1.0
due to two consecutive hyphens in a comment.
   <! -- sidebar.html -->
✉
Warning Line 104, Column 25: The document is not mappable to XML 1.0
due to a trailing hyphen in a comment.
   <! -- sidebar.html -->
✉
Error Line 107, Column 72: An img element must have an alt attribute,
except under certain conditions. For details, consult guidance on
providing text alternatives for images.
		<img src="http://www.gravatar.com/avatar/?s=150" class="img-circle" />
✉
Error Line 150, Column 67: Duplicate ID contact-list-secondary.
	<ul id="contact-list-secondary" class="list-unstyled list-inline">
✉
Warning Line 126, Column 53: The first occurrence of ID
contact-list-secondary was here.
<div id="contact-list-secondary" class="text-center">
✉
Error Line 162, Column 3: Bogus comment.
<! -- sidebar.html end -->
✉
Warning Line 162, Column 5: The document is not mappable to XML 1.0 due
to two consecutive hyphens in a comment.
<! -- sidebar.html end -->
✉
Warning Line 162, Column 25: The document is not mappable to XML 1.0
due to two consecutive hyphens in a comment.
<! -- sidebar.html end -->
✉
Warning Line 162, Column 26: The document is not mappable to XML 1.0
due to a trailing hyphen in a comment.
<! -- sidebar.html end -->
✉
Error Line 244, Column 360: The frameborder attribute on the iframe
element is obsolete. Use CSS instead.
…der-width:0 " width="800" height="600" frameborder="0"
scrolling="no"></iframe>
✉
Error Line 244, Column 360: The scrolling attribute on the iframe
element is obsolete. Use CSS instead.
…der-width:0 " width="800" height="600" frameborder="0"
scrolling="no"></iframe>