Are online wills valid?

​The answer is usually yes, but the simplest will can increase cost and issues during probate.
There are simple requirements for a legal will in Texas. 
The Texas Estates Code 251.051 states the will must be:

1. in writing;
2. signed by:
   1. the testator in person; or
   2. another person on behalf of the testator:

i.in the testator’s presence; and
ii.under the testator’s direction; and

1. attested by two or more credible witnesses who are at least 14 years of age and who subscribe their names to the will in their own handwriting in the testator’s presence.

Texas Estates Code 251.052 provides allows a holographic will to be valid with witnesses as long as the will is written wholly in the testator’s handwriting.
On the positive side, Texas provides allows a variety of simple methods for creating a valid legal will.  However, there can be a downside to writing a simple will that just meets the Texas requirements.
Attorneys and law firms with the proper legal forms and can help navigate around complicating issues.   The proper legal advice and forms can minimize estate costs and issues which ultimately decreases the total cost of the estate plan and execution.
Areas to consider:

1. providing for an independent executor (which reduces court costs),
2. allowing the executor to serve without bond,
3. properly structuring the specific bequests,
4. distributing residue of the estate,
5. disinheriting individuals,
6. leaving gifts to charities,
7. providing a no-contest clause, and
8. providing a self-proving affidavit which is signed by two witnesses and notarized (this eliminates the need to witnesses at the probate hearing to testify to the decedent’s signature on the will).

If you have minor children, you may consider:

1. a trust,
2. trustees, and
3. guardians.

To summarize, you may create your own Last Will and Testament in Texas with an online will. 
Proper will forms and legal advice will help minimize estate administration costs and issues.